No problem! I have an X-Fi XtremeMusic, but the Fatality shouldn't really be different at all. Mine is a 64MB X300 out of a Dell Optiplex that I was able to acquire from work. The 128MB X300SE version on Newegg doesn't seem to be much different width-wise.

Here's a pic I just snapped for ya:



As you can see, there's enough clearance between the X300's heatsink and the X-Fi card in front of it. An alternative which should work just as well would be to toss a PCI Express x1 card in the slot closest to the CPU if you ever want to run a Physics card. That will probably be what I do if I ever decide that a Physics card is worth it in the future. I hope I was able to help!