Personally, I'm an nVidia guy...I haven't used anything else since my first 3d-vid card (which was ATI)...From the sounds of things though, since AMD bought-out ATI, it seems like the quality of vid-cards coming from ATI is improving so who knows...I personally have no use for and no desire to run a Quad-Core...which is why I bought the X2 6400+...I usually don't have anything more than 2 memory-hungry programs running at once...the Quad-Core is more for REAL multitaskers...not to mention there are still plenty of games/progs out there that still don't support multi-core CPU's...if you're just building the PC for gaming, I'd say either stick with X2's or maybe even spend a few extra bucks for the FX dual-cores (maybe get a mo'bo' that can support Quad-Cores for future upgrades)...get the fastest memory that your motherboard can handle, preferably dual-channel as well. And if you're contemplating going to SLI/Crossfire, make absolutely sure your motherboard has TWO 16x PCI-E slots...a lot of them still have the 16x/4x configuration or other variations on the 2nd PCI-E...the last (and most important) thing you'll really need to look into, is how you're going to cool the system...the CPU and the vid card(s) do run hotter...I have to open nVidia Control Panel everytime I turn on my computer to turn up the fans on my vid cards just to keep them down around 60 degrees C (under load). They run a few degrees cooler now since I now have seven 120mm case fans sucking-in and blowing-out air, but I'm still looking to replace the stock fans on the 8800's but i gotta make sure the fan(s) will fit an SLI application.
One last thing on the CPU, if you're going to be using this computer for ANY heavy-duty emulation (ie. PS2), I'd actually suggest you look into Intel CPUs, because of SSSE3. All I know about it is that it really helps with the framerate. And since AMD stormed onto the scene, Intel has improved the efficiency of their CPUs significantly, so unless you're brand loyal, there's no reason to ignore them.