let's not forget a good soundcard will contribute a lot to system performance. try the crappy built-in soundchip versus creative vibra 64/128, & it actually accelerates your games. if u get yourself a new soundblaster card, the sound acceleration is even better, making overall system performance improve considerably.
regarding pentium & celeron, celeron's like pentium without accessories. the old generation celerons are pure crap but the new ones are ok. look out for certain speeds as some are good for overclocking. that is if u are interested in that. but if u don't know what i'm talking about, then my guess is that O/C is not for u. plus it's hazardous.
extra info; if u happen to use Win98/Me, u can always enable a switch in the registry that allows windows to make full use of your RAM space before resorting to hard disk for swap files. that way, for those with at least 128MB of RAM, u won't hear that blasted HD making noise anymore, that is until u start loading stuff from it. & that's one of the major factors that make me stick with the so-called crappy win98/Me. i'm a tweaker.