Actually... Newer blackberry's reach a critical level around 12-14 mb. So anything below that and it will not function properly. Curve 8900 and Tour users shouldn't worry since you have double what us Bold'ers have. I'd say you are quite fine having it remain above 50 mb.
Though I'm curious as to how it will get that low anyway. I mean I've got a decent set of apps and despite our ram memory total being only 128 mb, I usually have mine from 56mb to 38 mb after like 4-5 days use.
