I was reading on web some nForce 4 boards had difficulties with X-Fi but in 9/10 cases flashing mobo's bios solves the problem.
Asus A8N32-SLI Deluxe is not affected anyway so if only that stops you from getting X-Fi, I would not worry. I am very very glad with my X-Fi anywy: surround sound over regular headphones is freaking good.
But before jumping to X-Fi you could actually try to squeeze more from your onboard. It's in fact pretty decent, as it was noted above.