I recently upgraded my machine. I don't have my price list on me right now, but I know I spent right around $500 for everything I got.

eVGA nVidia 8500 pci-e16x 512MB DDR2
AMD Athlon 64 X2 4400+
Asus M2Ne-SLI mobo
1GB 667mhtz DDR2 (will be getting at least 1GB more after the holidays)
550watt PSU
250GB 7200rpm WD SATA HD

Vid card was on sale for like $120
psu was really cheap
Most of the cost was the CPU and mobo

My case is an old Antec Midtower that I bought when I last purchased a system. It has 3 fans (8cm if I recall), all simply pulling air out. System has plenty of breathing room, and temperature runs at about 93F for the CPU and 95F for the ram when not running games and such. It's not terrible, but I'd like to have more fans. However, the machine already is louder than my old one, mainly because of the larger CPU fan.

And I don't overclock.. though I have underclocked my cpu before. Old mach I had years ago was more stable when running just under the intended clock speed.