With 200 MB of "free" RAM, efficiency should drop somewhere near the 50 digit range, I guess.

Nevertheless, it would be an at least nearly optimal contribution.

If you have an Athlon, I'd suggest using ecm6.
In conjunction with prime95, the optimal B2 bounds for 50 digits (the current level for 24737*2^991+1) seems to lie approx. at 4e10. If you encounter thrashing, you could either use the "-k" parameter or a lower bound.
You could also try ecm on the other small k/n pairs. This way, 200MB are quite enough. I'm currently finishing the 35 digit level for all candidates with 1000 < n < 2000. For higher n's, Joe O wrote the current progress in the "Small n factoring" thread IIRC.

If you have a P4 or P-M, it would be a good option to let it run stage1 only with prime95. Afterwards, you could send the residues to another person, which does stage2 then.
Stage1 needs next to no RAM, so you won't have a disadvantage there.