You really have more options with a crossfire build honestly, just not one for under $500. Anyway I guess I'll take a stab at this.
ZOTAC NF750A-A-E AM2+/AM2 $89.99
http://www.newegg.com/Product/Product.aspx?Item=N82E16813500013
Athlon 5400BE $58.99
http://www.newegg.com/Product/Product.aspx?Item=N82E16819103289
Galaxy 9800GT $99.99
http://www.newegg.com/Product/Product.aspx?Item=N82E16814162028
Antec 300 & Antec earth watts 430W Combo $98.94
http://www.newegg.com/Product/ComboDealDetails.aspx?ItemList=Combo.193754
Western Digital Caviar SE WD3200AAJS 320GB 7200 RPM 8MB Cache $49.99
http://www.newegg.com/Product/Product.aspx?Item=N82E16822136098
Vista home Premium 64-bit $99.99
http://www.newegg.com/Product/Product.aspx?Item=N82E16832116488
Optical drive, monitor, mouse, keyboard, and speakers? Take them from whatever computer you used to type this message ^_^
And that gives you a total of $497.89 before shipping. Really though going with an ASRock 780GXE board that supports crossfire and a Radeon 4770 would be better as the 4770 is more on the level of the 9800GTX rather than the 9800GT. Two 4770s in crossfire will give you some impressive performance and boards based on the AMD chipsets will give you more CPU upgrade options and generally have better overclocking headroom than the nVidia based chipsets.