If you want SLI, then unfortunately you have to stick with a 680i or 780i chipset board..
If you want Crossfire, the P35 does work, but for true CrossFire Performance the X38 is deadly, it performs real good, and with the new Catalyst 8.3 drivers out, 3870 X2 Crossfire beats anything Nvidia throws at it.. Crossfire X is also an option, you can get 1 3870X2 card and one 3870 card, and Crossfire it.
Personally even though I run SLI, the P38 and X38 boards overclock better, and for the first time in few months or about a year, I see AMD ATI beating Nvidia at its own game.. So my suggestion stick with Crossfire.. Just my 2 cents..
But as your question, the answer lies in the first paragraph.. Good Luck