One significant addition to what others have suggested. If you want a very fast system for gaming, I very highly recommend for your hard drive, you buy 2 Western Digital Raptors in whatever size you can afford -- 36Gb ($100 +/- US each), 76Gb ($160 +/-US each) or 150Gb ($260 +/-US each) -- and put them in RAID 0 configuration. Even without RAID, these are the fastest, super reliable, non-SCSI drives available (10,000rpm). Use them to load your OS and all programs. Eventually buy a cheaper, slower drive for data, photos, music, etc if you use intend to use your system for such as well as gaming.
Mine computer is first a business system, not a pure game box, but the Raptor hard drives certainly are part of what makes it very good for gaming.