USB 3 is, for the most part, redundant for audio and are thin on the ground (or store). The "best buy" for quality and cost is the TASCAM UH-7000, which at $400 is as good as you are going to get as far as sound quality. Pro preamps and conversion. The downside is high latency on the usb 2 drivers and it don't look they will get an update. If you want similar quality (w/o preamps) the new lynx pci card - $700 to $1000 for a 2x2 or 4x4 unit each with analog and digital ins/outs. RME is another contender (good hardware and the best drivers).
But I imagine you are looking for something cheaper. Focusrite is a favorite here on the lower end, tho I expect the new TASCAM series will be good (Gibson owns Tascam and Cake), and Roland (previous owners of Cake) is another good choice. At the same price range all the hardware is about the same, except for drivers (and all the above companies have good drivers except the UH-7000, which are moderate.
I don't build computers - but pci slots are still valuable. They are like a van - they don't do you any good except when you do need them, and then nothing else will do. Try loading your mini with the show's gear. And power - for a few dollars more you are doing the best thing possible for the long-term service of your computer. Put something in those slots and you'll need more power, and for music itself the power supply is one of the differences between consumer and pro units.
have fun