Mixing ram for paired ram?

I have one Twinmos with winbond 512 mb 3200 cas 2.5 and I want to buy another but they no longer sell these so I was thinking would it be okay to mix another ram such as: Twinmos ram that is 512 mb 3500 or some totally different ram like a Kingmax for paired ram?

Mixing ram is always somewhat problematic. It's fairly safe to assume it will at least operate, given equal base specs, but consider this: even if it works, it will work together at the lowest common speed.
If that is no problem, you could simply give it a shot. If it all breaks down after all, then you could simply buy an additional stick of the new stuff and sell the old one... or you could do this straight away.

This guys sells it:
http://www.amdforums.com/showthread.php?s=&threadid=215882