SLI motherboards have a cost in addition to upfront $. They tend to fry OCd RAM, fry their northbridges and in the case of 680i, 780i and 790i can experience data corruption due to the MCP chip. Also factor in the cost of better case cooling. 3870x2 exhausts hot air from your PC, most 9600GTs do not.
If you value your time and a higher likelyhood of not having problems the 3870x2 (or even 9800GTX) is a better performer/cost. Otherwise, 9600GT in SLI is the clear winner.
I'd suggest waiting an estimated 4 months for a single GPU NV card. The newest ATI offering is widely expected to underwhelm.