Alain it can be a problem. But with the new arrangers it is not. If you are happy now, you will ALWAYS be happy. That's the difference between computer and music. If you have an old computer, you just CANNOT play certain software, games etc.
If you have an old keyboard however, you will STILL be able to entertain yourselve or people listening to you. If music is ones good, it will remain good. If people at a party like my sax, the will always like my sax. They will never say: "hey, the VA-7 has a better sax!!" . This is the benefit of keyboards over computers.
I have an Roland E-70 for 7 years now and I still can entertain people like hell! The sounds are still superb on a party. I need a new one ONLY because of the fact I cannot create styles myself.
So please don't worry to much. Start worrying about your pentium...