I use Deep Sky Stacker with 10MP raws from my 400d. DSS has stacked up to 53 subs with no trouble, and would certainly do more.
The only time I had trouble was when I selected 'Bayer Drizzle' instead of standard debayering. Bayer Drizzle is meant for small, webcam-size frames, so asking it to drizzle process 10MP raw frames was a bit much really.
I run DSS on my laptop, it has 2GB ram and windows XP.
Cheers,
Rob