my rule of thumb nowadays is if you are two generations behind, it's really time to change...
there's pIV now... you have a pI
it's really time
hehe..
I wouldn't go for anything under a pIII 800ish....
I'm about to move myself, got an old celeron 300mhz, O/C to 450... but hey, it can still wait..
build it yourself man... it's not that hard... go to sharkyextreme.com or tom's or whatever, there's build-your-own-computer walkthrus... you'll save a few..
you can get a good deal online...
pIII 800s for like 300$, 2X256MHZ for 100$,
then the rest depending on what you want/need ....