At the moment, I have 2 Alpine 10" S-Series subs hooked up to a 400W Audiobahn Amp. (Regardless of what my sig says; I'm upgrading soon. Each sub is rated at 300W RMS, so I'm going to go with a 600W amp.) I can tell you that the bass on this setup is pretty rediculous. My car rattles at times; the licence plate (need to get some foam padding under there), the rear-view mirror, etc... Soon it'll be pumping out 150% of what it is now.
It all depends on what kind of music you listen to. For rap/r&b/other bass-heavy music, you may want to go with a pair of 12" subs for even more power. But for this car, and especially if you listen to rock of any sort, two 10s would be great. As for brand-names, I personally like Alpine, although I can't afford Alpine amps. Audiobahn is good, and I've heard that MTX puts out good products. I'd stay away from Sony, but that's just from what I've heard, I don't have any personal experience with their amps or subs.
Make sure that you also upgrade your speakers, if you haven't already. ATM I still have my stock 17 year old BMW speakers; I can't wait for my Alpine 6x9s to arrive.
Good luck with your system, i'm sure it'll be "bumpin" soon.
My 1987 325 is no longer. Rain and an relatively inexperienced driver are not a good combination, to say the least. Luckily no one was seriously injured.