It all depends on what you want for quality and everything. First off, cassette recorders use 1/8" inch tape. If you want just a couple mics at the back of the room (or a stereo mike at the back center) you should be able to get a good sound, and after listening back, adjust your amps and everything until you get a good mix. This will help ensure that you all will sound good together live. As for what to record it to, it's all in what you want to spend. If you own a laptop, you could record direct into that and save it to the hard drive. This is a good option if you already own this piece of equipment.
If you already have a cassette recorder, you could use that. Quality between recording direct into your computer's sound card or on a cassette recorder should be halfway decent if you set things up correctly. Only problem with cassette is you will have to turn the tape over, even with an auto mechanism, there would be that brief few seconds where nothing is being recorded as the heads on the recorder move. Cassette might be an option if you want to practice for say 45 minutes, break, then turn the tape over. But, tape will cost you a few dollars each practice, whereas you could burn a CD of each practice with a computer for really cheap money (not to mention make copies that you could give to your bandmates. I do have a casette that will change sides and keep recording on its own in a loop, but like I said, there is that break of a few seconds where it changes sides and goes over the leader tape where it won't record. My cassette deck is in an old Hitachi FX-7, but this would have much more on it than you'd need (CD player, radio, two cassette players, etc.).
Minidisc will also yield decent results, but again it's the cost of media each time and buying a minidisc recorder. Unfortunately, they don't usually put mic inputs on standard consumer minidiscs recorders anymore.
If you own a laptop, or an old computer that you don't use anymore, maybe you'll want to set it up in the back and record that way. It wouldn't take a powerhouse to record at CD quality (16-bit/44.1k) and get decent results.
Another option is to record to VHS tape, if you have a spare VCR and mixer you're not using. There should be a way to do this really cheaply and you may already have the correct parts at your home. I wouldn't recommend going direct from your PA however. Rather, mics at the back of the room would give you a better live sound, not to mention you'd actually hear how the audience will hear you.
Best of luck,
-MD
