256M is enough RAM to do most things today, unless you are a pretty diehard gammer. My video comp and my laptop(which I do a lot of video and picture editing with) both only have 256Ms and do nicely...
Anyway, no reason you can't upgrade
I don't think it really matters. I have heard somewhere it's better to have one larger chip, and that's how I tipically upgrade my comps. However, as long as the ram speeds match and all, there shouldn't be a problem.
I guess that's all that oculd really cause problems. If the chip speeds are different. Not just the PC2100 and all, but the ns's of the chips.