Just to mention this as others have. Do not buy from Tigerdirect. I recommend Newegg.com and Zipzoomfly.com to buy your items from. Both are great companies and easy to deal with as well as lower prices. This will save you some additional money as well in most likelyhood.
Newegg has a cult like following in the computer builder circles as they are great to deal with and ship items out rather quickly as well as the generally lower prices.
And as others have said, 500w for a PSU should be just fine, but if you can get a quality 600w for $40, go for it. Having too much is better then having too little. But if you can find a 500w for less, go that route. AMD vs Intel is all about personal preference. Even though that the newer Intels can do some things better then the AMD's, the cost difference usually pushes me back to the AMD. Graphics card wise? Do what others have suggested, get a better mid-range graphics card and just a single one. only 1 out of 10 games really seem to take advantage of SLI or Crossfire.
With your motherboard, just make sure you get one that is reliable and from a reliable manufacturer. RAM, as long as it's compatible and at least 2gb, you will be fine. I'd recommend going with 2 (SATA) HDD's and run them in Raid 0. Raid 5 is doable as well, but 0 is fairly easy to set up for someone who may not have as much knowledge. When you have it set up that way, installation of XP is quick and easy. And yes, if you're gaming, definitely get XP.
My pokemon brings all the nerds to the yard. And they're like, "You wanna trade cards?" Damn right, I wanna trade cards. I'll trade this, but not my charizard.
