I'm not the complete sound card expert, but you can check out the Computer Recording and Sound Card forum, and/or do some searches.
I've heard very good things about the Aardvark Q10 (the one with the buit-in pre-amps), but it's a bit pricey.
I'd say the preamps in your Mackie would work very nicely for a card without preamps, but if you're going to record live, won't you be using that mixer for your stage setup? I don't know of a way you could set it up for double-duty because you'd need to preamp your sound card using your Mackie's channel inserts.
My guess, is that you've done some research, here's what you're gonna find.
A) If you can use your mixer solely for recording and don't need it for stage setup, you'll probably want to get a Delta 1010 or a C-Port. The Delta is a little more expensive of the two, but a little higher quality (that's the consensus, anyway). However, the C-Port is easier to add more inputs.
B) If you can't use your mixer exclusively for recording, then get an Aardvark Q-10. I've been told the preamps are very good. I'd stay away from the Pro-Tools DIGI setups - they're designed for Macs and buggy on the PC (somebody's gonna jump all over that one
Please understand, these suggestions are my opinion, and I'm sure someone will disagree based on different experiences. If you're looking to record as many channels as possible though, don't skimp too much on the PC. Make sure you've got a fast CPU (at least 1ghz) and at least 256mb of Ram, and a big hard drive (at least 40gigs).