Hello there. Just to add a few specifics:
1. MAudio's midisport 2x2, 4x4, etc. all work with reliable drivers. Operating systems--most any version of Windows--cost is anywhere from 59 USD to 100+ USD. This also works well with Linux. (I use it with opensuse 11.3,(64 bit and actually faster than running the same machine and software under windows...some things.) and about 90% funcionality, and more working every day.)
2. Roland's UA-4fx, etc. All work with Windows XP, Vista, Windows 7...the drivers might have to be searched for under windows, but they are there and will work well. Most of them work with Linux out of the box. There is built-in support for the UA-4fx right in the kernel. But DO read the advice about setting the switches on the box before it will work. Got a problem with Linux? You can email me, or Facebook, or right here and I'll do what I can to help you. This Roland box does a lot of good things for you--midi, audio (with effects), and can be set as both the midi and audio interfaces under options...Biab and realband both recognize it, and Power Tracks if you need it. It is as faithful as old dog trey on my crappy old homebuilt. It has all the appropriate connectors for both midi in and out, and includes an XLR connector for high-quality mics, and can provide 45v phantom power if you have a condeser mic. Did I mention that it connects through USB, and is designed for USB-2.0, and can also use USB -1 if that's all you have. Cost is about 129 USD.
3. The Tascam USB 122 can be made to work, but can be tricky if you aren't a geek, It's a nice box, but wouldn't recommend it. Having said that, It actually works better under Linux than Windows, and also does MIDI and Audio. I think it's about 100 and change for price. Some of these will work under Mac, I'm told, but haven't used them on one, so I can't say much. I've used all of the above mentioned, and will help you if I can.
Ciao.

Hope this helps somebody.
Joey45