Sound Cards for the Mac Pro

I just recently go a new MacPro...fully loaded with 3gigs of ram and two 500gig drives extra. Also a copy of Windows Vista Untimate which I installed with Boot Camp. I'd like to take advantage of the extra PCI slots by buying a sound card that would give me a boost on sound performance. Does anyone have any idea which card I should buy and/or which ones I should avoid? Thanks in advance for any help!