well, this doesn't include the psu, but I did something similar, going from a Celery 2.4 with 1 gb pc3200 and a radeon 9800pro.
Asrock 4coredualvsta (I got the 775 dual, but this one lets you use a 4 core proc later for an extra $2.00) $60.00
The board has ddr and ddr2 slots (can't use both at once though) and agp and pcie (4x only) for video.
Intel e4300 dual core 1.8 $169.00
The motherboard lets you reuse your ram and video card. I promptly overclocked it from 1.8 to 2.4 on the stock heatsink/fan. But I already had a 380Watt psu for my radeon so.... you would have to spend a little more, more like $300. But I didn't have to buy new ram, new vid, etc...
If the prices on the procs drop next month or so, you may be able to get the proc for much less, so maybe $200 would be possible.
Plus, you can buy a quad core if you wanted next year, or a pcie card (the motherboard has both agp and pcie) the pcie only runs at 4x, but iirc from all the articles I've read, it didn't make that much of a difference with the last batch of cards.
Centurion