I'm in the same boat actually. Very similar system too. Kind of wondering if it would be better to just add another GTX 275 or wait for GT300 cards.
My system is:
Antec 900
Core i7 920 (2.66GHz Overclocked to 3.35GHz on stock fan/heatsink)
3GB (3x1GB) Corsair XMS3 DDR3-1600
eVGA X58 3xSLI LE...