Well, if you're looking for bang for the buck the Yamaha range of USB mixers looks good. The starter is the MW10C, it would let you connect direct from the line out of your amp and a microphone simultaneously (that might be useful for blending in your mix), it has a lot more ins/outs/loops than a similarly priced audio interface and it also comes bundled with Cubase AI4 which should be a bit more flexible than Audacity.
That said, I haven't tried one so can't vouch for ease of use, sound quality, etc so check out some onlline reviews.
Personally I use an ADA preamp and a Line 6 POD XTL through a Presonus Firebox into my iMac and with the exception of my Firebox living up to it's name last summer, I've never had a problem.