Okay. I honestly don't buy into quad core being that big a benefit over dual core YET. Applications just aren't that multithreaded - and I could go into rather excruciating detail about what I mean by this if you really wanted... but won't for now. I've got a nice Athlon 64 X2 (dual core) and even with LotRO running with most settings on their highest, I don't max the CPU while I'm playing. About the only time I max the CPU is when I'm doing a gaussian blur on a photo from my 10 megapixel camera, or maybe when doing complex video transitions for a DVD I'm authoring.
With that out of the way, let me say this: the real bottlenecks in gaming these days are twofold: data access speed... and the graphics card(s). With that in mind, let me answer your specific questions.
1. As said above, I personally say a good dual-core is just fine. Unless you want to plan for possible future applications, I wouldn't bother with a quad core. I don't believe that games really need that much CPU horsepower when the GPU is what matters more. I know Intel has the better performance, but I like AMD's prices. But that's just my preference.
2. I personally hate SLI. You double your cost for maybe 50-75% increase in performance. I run on an 8800GT and run most everything on ultra/very high and see reasonable frame-rates. If you get something in the 9800 series or, as you mention, a GTX260 or 280... you'll be doing JUST fine.
My gripe with SLI is that later, if you want to upgrade your graphics, you've got two cards to replace, not just one. I much prefer single-card solutions. Unless you "need" enormous performance for something like a competition-level FPS game session, I just don't see where SLI is worth it.
3. I'd be more concerned about the speed of the memory than which DDR technology it uses. Make sure you're going with a dual-channel solution and a good, high clock speed on the RAM. In general, I'm sure DDR3 is faster than DDR2, so that should be better.
4. Well, if you were to get a GTX 280 for your graphics card, you'd need at least a 550 just to support that video card. However, you may not need much more than that. There's a couple places out there that have some nice
power supply calculators. Use that to get a better idea what you need than just asking people.
Though I would say that whatever that calculator says, add 100w to it as it's telling you the -minimum-... and having excess wattage available is not a bad idea. Also, do pay attention to what the video card says it wants as a minimum - that usually has to do with the absolute peak power requirement for the video card.
More importantly, you might want to consider looking for one of the 80+ power supplies - the more efficient it is, the less power it will use from your wall. And make sure to get a good, reliable brand PSU. I like Thermaltake, myself. Dirty power can wreak havoc on motherboards.
5. I run on Vista Ultimate (x64) and have no problems at all. For amount of RAM, I disagree with Thel on this. With Vista 64, you'll see a HUGE performance gain going from 2GB to 4GB. Also, LotRO is a memory hog, if you let it be - loading textures off HD to RAM for faster access. There's a specific setting for how much RAM to use for texture cache in LotRO.
However, Thel is accurate in that LotRO is a 32-bit application, but the benefit that a 64-bit OS provides at this point is in memory access - it lets you go beyond 3.5 GB (without doing weird OS tricks) and really see a benefit from the extra RAM. I'll say this point blank.. Vista 64 REALLY WANTS 4GB+. Oh, and I've never, ever had any problems with Vista Ultimate x64 recognizing and accessing USB storage or other devices quickly.
6. I've never overclocked, so I've never had any problems with stock coolers on the CPU or GPU. I usually look for cases where I can put both a front and rear case fan on. And then look for fans that run reasonably quiet. Might want to choose a case with a filtered front bezel if you can find one. It helps cut down on dust, after all. DEFINITELY use noise-reducing gaskets on your fans. Oh, and look for a PSU that has two fans in it too.
On cases, I really, really like the one I have, though it only fits MicroATX boards (which is fine by me with my single-GPU, two-drive setup).
CoolerMaster Centurion ... It mounts the motherboard upside down, on the left side of the case, putting the CPU at the bottom (with the coolest air) and the GPU up at the top, closes to the PSU, which vents most of the air out the top and back. The GPU is usually the thing that produces the most heat, so I find that design makes sense to me.
That case would NOT work if you want A: a full ATX board, B: more than two hard drives. I also like that it's smaller, which means it's easier to take to the LAN party I attend.
Now... Last but not least... 7. The unasked question. Hard Drive(s).
First. Never, EVER get anything less than a 7200 RPM drive on a desktop machine. These days, I highly recommend SATA 2 (That's the 300 Gbps data rate). More than that, I definitely want to prod you to consider a dual hard-drive setup. Here's the reason behind this...
Windows has this thing called the Page File. It's a big chunk (usually a bit more than however much RAM you have) of HD space where Windows pages bits of memory off from RAM that aren't used as much as other bits. This way you can use more RAM than you really have - because it only keeps the bits it needs in physical RAM. This technique is commonly referred to as "virtual memory". And you don't want to turn it off as it actually means better performance in certain ways.
Now, the problem with the Page File is that it requires a lot of reading and writing when you're using a lot of RAM or when switching between big applications. So, if you've got your HD busy doing read/writes for the pagefile, it can't spend that time doing reads of things like, say, huge volumes of textures... Like, oh... I don't know... Walking into Bree when the Hobbiton Philharmonic is holding a concert outside the Pony.
Fortunately, it's easy enough to put in a second hard drive. With SATA, each device is on its own "channel", so the machine can access both drives at the same time. This lets your primary hard drive do all its fast read/writes to the pagefile while the secondary disk is reading textures like mad.
So... Drive C: Windows and generic applications... Drive D: with LotRO and other disk-intensive applications... and your other data.
For the record, using two hard drives like this is exactly what the whole
FlashBoost idea is trying to do for single-HD systems.