ShlupMoo: I suggest trying a different BT client if your current one is giving issues. I find Azureus is pretty good, altohugh being Java based, I find it's using around 70 MB when running maybe 500 MB worth of torrents. I've only had Azureus go berserk with eating memory when I had another process doing heavy disk work, so Azureus couldn't flush to disk fast enough, so I ended up with a few hundred MB used. It was nice to see how fast that cleared though when the other process stopped, since it looked like it just did a few hundred MB in a seconds on percent complete bar
As for a program needing 700 MB active at one time, a large heavily used database could do it, or complex simulations or modelling

Or virtual PCs, I'd rather they have all their virtual RAM allocated in real RAM, since they're slow enough as it is without the extra overhead of the host OS swapping memory about all the time