Here's a system I recently built for a friend :
AMD Phenom II X4 955
MSI Hawk Radeon 5830 1GB
4GB Elixir DDR3 1600MHz
1xSSD 32GB SATA 2 OCS + 1x500GB Western Digital Caviar Green
Chieftec 500W PSU 80% certified
He had the case already so I did the mounting. Performance wise, you can't get any cheaper. The whole thing cost about 2400RON which is about 750US dollars. You may get cheaper prices than this, depending on which part of the world you're in or if you want to sacrifice L3 cache for L2 and go for an Athlon or even take the SSD and GPU out but what I listed above is a system that will hold a couple of years even in demanding games with AA and AF at max at around 1280x1024. That's my friend's maximum monitor supported resolution so I built the system around that and around his wallet
You don't really need a six-core unless you are absolutely sure that the programs you are using support multi-threads. The best way to go is a quad because 1.5MB L3 cache per core is better than six cores with 1.0MB L3 cache per core. In applications such as Photoshop and other thread-intensive stuff, more cache is better than having more cores.