I have a question regarding mixing ram. I was wondering if I could mix 2100DDR ram with 2700DDR ram together? The mother supports both , but im not sure if its ok.
There's no definite rule. Most often, it will work just fine. However, sometimes two different sticks don't play nice together. The only way to know, is to try.
Your RAM will just run at PC2100 if you mix it with the PC2700, since the PC2100 can't run at PC2700 the mobo underclocks the PC2700. Unless you have a dual board in which case I suggest you get a matched pair.
Since the mobo will probably be downclocking it anyway, you may want to buy 2 sticks of higher speed RAM (if your motherboard supports it) or just buy a stick for 2100 for really cheap.