Make sure that the disk drive and hard drive you have are SATA and not IDE.
Well, these will save some $$.Corsair 400W psu
Cheap, good quality psu
$50 ($40 after MIR)Rosewill micro atx case
Cheap case. That is what you wanted, yes?
$25Corsair 4gb DDR2-800
Good, cheap ram
$45 ($20 after MIR)AMD 2.9ghz dual core
Will outperform a 2.1ghz 3 core processor in most things
$80Sapphire Radeon 4830
If you want some kick for gaming, this is a decent card. About the equivalent of a nvidia 8800gt/9800gt
$90 ($75 after MIR)GIGABYTE GA-MA74GM-S2
The motherboard you picked
By my math, total = $295 (After mail in rebates, before shipping)