Assuming you are going to re-use your case/ power supply/ hard drives etc etc then probably the best value system for SLi might be this new mb from MSI
MSI NF750-G55 AM3 NVIDIA nForce 750a SLI $100http://www.newegg.com/Product/Product.aspx?Item=N82E16813130235
Visit the webpage for the board at http://tinyurl.com/lzundm
AMD Phenom II X3 720 2.8GHz $119http://www.newegg.com/Product/Product.aspx?Item=N82E16819103649
Patriot Viper 4GB (2 x 2GB) 240-Pin DDR3 SDRAM DDR3 1333 $85http://www.newegg.com/Product/Product.aspx?Item=N82E16820220333
So mb/cpu/ram for $305
The motherboard is fairly new so there arent any good reviews I could find . SLI is pretty new for AM3 so expect the drivers to be improving for a while .