don't know which GTX you choose, but if it's standard dual slot, there's no problem to put the X-Fi in the free PCI-e 1x slot remaining. I'm using 2 MSI 8800GT Zilent in SLI, they are dual slot, I placed them in the blue slots and there's enough space to put the X-Fi between them.

@ theYipster : any answer to my question about RAM timing and frequency ? if i'm right, the fastest, the best (so I would say 1000Mhz@5-5-5-15 better than 800Mhz@4-4-4-12). but I prefer an expert to tell me...