This is exactly the question i'm asking myself at the moment.
I can't afford to get the 7800GTX, so i'm thinking about getting another 6800Gt second hand and going Sli.
I can't really see any disadvantages.
Anyone know where I can see some benchmarks of an SLI rig.
I've found this one:
I would recommend just getting a new graphics card. If you add the expensive of a new motherboard and then a 6600 it's not worth it for the performance your going to get with 2X 6600.
If I were you then I would get a Sapphire ATI Radeon X800 GTO2 256MB DDR3 TV-Out/DVI (PCI-Express) £158 and then overclock it to a X800XT.