I myself wouldnt put any money into that system. Those old Pentium 4 chips were really not very good even when they were new.
Honestly you could upgrade to a low end newer dual core system that would blow that old one away for a few hundred dollars.
To me any small speed increase from updating the old processor is NOT worth it. Since you already bought an agp card ( obsolete since 2005 ) I would just make do with what you have untill you can afford to put some money into it.
When you do want to do a better upgrade post what the budget is in the New System Build section of the forums and people will help you get the most "bang for your buck".