Here's a build. It comes to $569. You could go cheaper on a few things, but not by much. I chose the particular video card over cheaper ones in part because it exhausts most of its heat out of the case. Another $20 would buy a socket AM2+ mobo, which wouldn't be a bad idea if you decided to grow this rig as better parts become affordable. The PSU is the minimum I would use on this build. It doesn't include a keyboard, mouse, monitor, or surge protector.
Rosewill R604-P BK 120mm Fan ATX Mid Tower Computer Case - Retail
Model #: R604-P BK
Item #: N82E16811147058
$39.99 -$5.00 Instant $34.99
GIGABYTE GA-MA69VM-S2 AM2 AMD 690V Micro ATX AMD Motherboard - Retail
Model #: GA-MA69VM-S2
Item #: N82E16813128043
$59.99 $59.99
HIS Hightech H195PRQT256DVN-R-V2 Radeon X1950PRO 256MB 256-bit GDDR3 PCI Express x16 HDCP Ready CrossFire Supported Video Card - Retail
Model #: H195PRQT256DVN-R-V2
Item #: N82E16814161088
$169.99 $169.99
FSP Group (Fortron Source) AX450-PN ATX12V 450W Power Supply - Retail
Model #: AX450-PN
Item #: N82E16817104954
$57.99 $57.99
AMD Athlon 64 X2 4000+ Brisbane 2.1GHz Socket AM2 Processor Model ADO4000DDBOX - Retail
Model #: ADO4000DDBOX
Item #: N82E16819103774
$65.99 $65.99
A-DATA Value Series 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Dual Channel Kit Desktop Memory Model VDQVE1A16K - Retail
Model #: VDQVE1A16K
Item #: N82E16820211165
$69.99 $69.99
Seagate Barracuda 7200.10 ST3250410AS 250GB 7200 RPM SATA 3.0Gb/s Hard Drive - OEM
Model #: ST3250410AS
Item #: N82E16822148262
$69.99 $69.99
LITE-ON 20X DVD±R DVD Burner with LightScribe Black SATA Model LH-20A1L-06 - Retail
Model #: LH-20A1L-06
Item #: N82E16827106072
$39.99 $39.99
Subtotal: $568.92