To test out your RAM, I'd usually put in 75-80% of your total RAM into it ...
For eg, if you have 1GB, I'd usually put 750-800MB there ...
But then, if you're running dual core, I'd put 375-400MB per instance ...
75-80% along with your applications running, would usually take up 95+% of your RAM, and that's a lot of stress on your RAM ...