"""is it worth getting a new motherboard and buying a secondhand 8800GTX for SLi"""
as a 8800gtx owner i say no!
you will still own old tech its better to buy new tech either ati's new cards out now, or wait till the g300 nvidia cards come out in 2-3 months
"""Which would run games better?"""
at the mo. for best graphics and speed combined to furture proof yourself for 2-4 years get the ati 5870 (as im about to as well)
ps what monitor and res do you use?