Others have already explained it but let me clarify. To run 2 graphics cards, you need, obviously, 2 cards & a motherboard which can support them.
If you run 2 ATI graphics cards, this is called CROSSFIRE & you need a crossfire supporting motherboard.
If you run 2 NVidia graphics cards, this is called SLI & you need an SLI - supprting motherboard.
The two standards are not interchangable.
You have 2 NVidia Graphics cards, paired with a CROSSFIRE motherboard. So no, they are not compatible.
If you must run a 2 graphics card setup, then you must either change those 2 NVidia 8800GT graphics cards for 2 ATI 3870 graphics cards, or change the motherboard to an SLI one such as a 680i motherboard.
I'd question whether 2 graphics cards is really good value for money or necessary anyway.
One of those 8800GT's will play almost any game at highest settings at any reasonable resolution & will continue to play new games for some time to come.
If you are not using 2 Graphics cards, then a motherboard based on the P35 chipset (the one you have is X38) will be in every way almost as good for around £80.
I agree on the RAM. There is almost no compelling reason at all to pay more than about £80 for 4GB of decnt RAM.
Overall, this specification contains many good parts but represents poor value for money.
The processor (Q6600) is spot on, probably the best value for money currently available & will last for a long time.
I think the case is a bit expensive & not the best looking but the main thing in a case is if someone likes the way it looks, so if he really wants that one, then go for it.
If you have a large budget, the extra would be much better spent on a nice, large, qulatiy monitor which will make using the whole computer much more pleasurable & last for years to come.