there are no good $500 sli builds . They'd have to be based on nVidia chipset mb's and they dont have a great reputation .

You can build a great pc for $500 [ depending on what you actually need to buy ]
and you can find those recommendations by scrolling back through this forum and looking at all the thread title that have "$500 " in the title
 
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.
 
Ooops, forgot the RAM so add

OCZ Value Series 2GB (2 x 1GB) $21.99
http://www.newegg.com/Product/Product.aspx?Item=N82E16820227060

Bringing your total to $519.88. Yeah I tried. Anyway you could go with a 9600GSO instead to make budget like this one

XFX PVT96OS1S4 GeForce 9600 GSO Fatality Edition 768MB $66.99
http://www.newegg.com/Product/Product.aspx?Item=N82E16814150358

Which would bring your total to $486.88 so you could have a little money left to get whatever optical drive was on sale ^_^.
 

Change CPU to this - It is better than the 5400 and is newer...
http://www.newegg.com/Product/Product.aspx?Item=N82E16819103300