Well my friend gave me his pc 2100 128mb stick and i wanted to stick it in my extra ram slot, currently i have a pc 2700 ram stick in there will there be any trouble? I heard you could but it will be slower? So my question is, is 2 sticks together totaling 640 at 2100 better then one at 512 at 2700?

The motherboard will run at the slower memory's speed, but I don't think you will notice any slow down...

"So my question is, is 2 sticks together totaling 640 at 2100 better then one at 512 at 2700?"...

No

More is better, but whatever you add will work at the speed of the slowest module. You can actually use pc2100, pc2700, pc3200, or PC4000 on most motherboards... but not all FSB sizes will accept all at the same time.

Thank you.