I have a macbook 2.0GHz, 512MB ram. Factory specs. I want to upgrade to a single stick 1GIG ram, but I would like to keep one of the 256MB ram to have a total of 1.25GIG ram. Is this ok to do? Are there any side effects?
It's perfectly OK. Matched RAM runs a tiny bit faster, but more RAM almost always trumps paired RAM. Go for it. And since you wouldn't be pairing the 1GB stick, there's no reason whatsoever not to keep the extra .25GB.
Having matched RAM will provide you with the best possible speed, but certainly bumping to unmatched 1.25 GB from matched 512 MB will give you a huge boost in performance.
My MBP is running a 1x1gb stick and a 1x256 stick, and it runs fine. It seems slightly slower in my opinion than the 1x1gb alone, but things tend to run more smoothly.