1) The phenom is an amazing processor (especially for the price) but with $1300 you could definitely step up to the i5/i7s.
2) The GTX 275 has been replaced by the Radeon 4850
. Its much faster for around the same price.
$140 Gigabyte Motherboard
$200 Core i5 750 Processor
$85 Gskill Ripjaw DDR3 2x2gb
$100 Antec TruePower Power Supply
Case + Harddrives are up to you. Getting more than 4gb is not really necessary. I've played Crysis ultra high settings on 1920x1080 and I used just over 3gb.
Yeah, it's simple. Just plug everything up. Turn on your computer, put in the Windows cd and go through the whole setup process. After that's done it will restart and you'll boot up to your desktop. At that point the first thing you need to do is install updated drivers for your hardware off the internet. Mainly your graphics card and sound card (if you have one.)