When you are ordering parts, www.pricewatch.com will give you a list of prices for the items you want and all of the websites they list are reliable. If you want to "trick out" your puter check out www.xoxide.com. When going with brands, and this is just my personal taste, I prefer AMD (make sure it's a barton core) over INTEL and ATI (don't get the SE kind) over NVIDIA (ati is generally cheaper and just as good). Since the 64-bit stuff has been out for a while you can get some of the older chips and motherboards in combos for around $100. Alienware claims to make PC's built for games but, they are hella expensive. Anyway, here would be a "beyond the top of the line" pc:
$81 - Athlon 64 FX motherboard
$1049 - Athlon 64 X2 4800 processor
$500 - RADEON X850 XT Platinum PCI Express
$838 - 2 Serial ATA 500GB (RAIDed for a total of a TERAbyte and 14400 read speed!)
$950 - 2 PC4600 DDR 2GB RAM sticks
$3418 - Total (without a dvd drive, fans, case, etc.)
If you are not looking to spend $3500+, you could try this:
$261 - Athlon 64 3400 with cpu/motherboard, fan, 256DDR pc3200 RAM
$118 - RADEON X700 PCI Express 256mb
$55 - Serial ATA 80GB 7200 rpm
$434 - Total
Hope this helps.