The OS X x86 (on Insanely Mac) guys have a Wiki on sound cards and probably would be more helpful in answering this question. From what I've read, the Creative sound cards work but only in AC97 mode essentially. Some other "better" sound quality cards should also work depending on the chipset used, the hard part is finding a PCIe one (I found three on Newegg, two were creative and the latter was an ASUS card based on a realtech chipset but with a DSP for CPU offloading... in other words not going to work without help from ASUS). Sound cards almost always lack a ROM of their own so EFI v.s. Bios is a moot point in theory. All USB audio cards that conform to the USB audio spec should work just fine.
That said, the built in audio isn't all that bad unless you need 5.1 or something special, etc. You're not going to find any hardware acceleration for the Mac (Creative made one half caring attempt at this and creating an expensive and unpopular option for the Mac). Most cards use chipsets from three different manufacturers anyways (Realtech, Via, and CMedia - the latter being what Apple and a lot of other companies use) so you're just going to be buying better filtering, circuitry, etc.
None the less you may want to ask the OSX x86 guys for their input on the matter.