Depends on what you're doing with it, but you may or may not notice a 256mb response. If you're doing some power-computing, or heavy gaming, then yes. Otherwise, if you notice the change, you're lucky.
Why only buy 256mb though? I mean, RAM is so dirt cheap now. 512mb wouldn't set you back much further...unless you buy that over-priced "SUPER ULTRA SUPREME DELUX LL LIGHTNING" stuff. Besides, buying super-fast RAM won't do a thing for you if your current RAM is slower than the RAM you're buying.