Use the SSC version at newegg for sli, $160.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814130409
In sli, your older card will drag down the new factory overclocked card back down to its own level. You can always overclock the slower one. Other than that, it'll work fine.
As for buying a 9800gtx+ or gtx260 instead, it's a bad choice. The sli setup will significantly outperform those single cards for less money.
The g80 8800gts 640mb performs about the same as 9600gt, which cost $100. So it's not a good deal for single.
http://en.expreview.com/2008/04/02/g80-vs-g92-hi-end-geforce-gfx-cards-round-up.html/10