My experiences with SLI in WoW were horrible. Real pain the ass honestly disabling 1 GPU just for this game in order to get decent fps :\ I'd really recommend just getting a gtx 275 for your needs
Personally I just bit the bullet and am upgrading my pc from a core2duo to the i7 platform. Newegg open box deals were too good to pass up! My reasoning is that the i5 motherboards will essentially be priced around the same range :)