You are wasting you money with SLi and FSX unless you are running a screen resloution of 2xxx and above, and, AA at 16-32x
With crossfireX its new and although the benefits are a bit better than SLi, its really not worth the cost
Right now purchasing is BAD. Everything is about to get changed this summer, when exactly I dont know but purchasing right now will limit you to the video cards and processors available today for full support.
The 8800GTX and its variants with the 384bit memory buss are the best performers but as always with Nvidia there are usually drver issues as they release new ones practically once a week. With ATi there are less driver issues but they can not out perform the 8800GTX and its variants
I have not been involved with hardware testing for several months now however I am expecting to be testing a AMD 7 series along with a Intel Nehalem/motherboard later this summer.
Nehalem processors will have from two to eight cores and include up to 8M bytes of level 3 cache. No motherboard on the market right now will ever support it. It requires an entire new platform to run it and it will require DDR3
I would not purchase SLi if you paid me for it
Your best bet if you intend to purchase now is either a x38 or x48 chipset board. If you are looking for the best performance than that would be the Nvidia cards that support 384bit memory buss.. the 8800GTX is still the best bang for the buck even with all the BS 9xxx upgrades they put out there.