Parts all come from a US-only site called Newegg.
They sell every manufacturer of computer parts at the best possible prices and have amazing service. Also every part has extensive customer reviews so you'll always know how something will work when you get it.
Motherboard for $100 (LanParty, Asus; something that suits your needs)
CPU for $130 (which includes heatsink and fan) (some 3.0 GHz CPU, like the AMD 6000+)
HD for $80 (Western Digital 250GB)
Memory for $100 (Anything. I use Patriot DDR800 that I got for this price)
Case and PSU for $70 (I use some Alienware case that I scored a deal on, came with a 700W PSU)
DVD-RW drive for $45 (Optional)
Video card for $180 (EVGA 8800 512MB card)
Total is $705 but there's a good chance you can simply use the DVD-RW and Hard drives from your current system, which is what I always do.