It doesn't look like you're going to have room for that PCI-E X-Fi Titanium, obviously due to the graphics card.
I've always been partial to sound cards but today's onboard sound has improved dramatically over what it once was. With that, have you auditioned your onboard sound and decided that the quality is below what you'd like? If so, and you definately want a sound card go with something PCI and you'll be fine.
Sound cards don't really benefit from the PCI-E bus (unless something has radically changed) so a PCI audio card would do you justice:
Again, check your onboard sound and see if you really need something better (didn't think I'd ever be saying that), ...and if you don't have really decent speakers or headphones than you may not even be able to hear what you've paid for in a discrete soundcard.
The reason why I want to buy a sound card is because I have the same speakers as a friend of mine, his computer is sort of like mine. The only difference is that he has a sound card, and his speakers sound AMAZING compared to what I hear on my computer.