1) What are your current specs?
2) Is emulation all you use your grfx card for, or do you do other PC gaming?
If you have a better grfx card than CPU or vice versa, upgrade the worse one, as it's most likely a bottleneck for the other. For general PC gaming, I think a good grfx card is really more important, unless your processor really sucks. For pure emulation, though, the CPU is more important.
Just my opinion, though. I'm not the most knowledgeable guy when it comes to hardware.
Yep.. Emulator use a lot of cpu power.. I once had an amd k6-2 533 mhz with a voodoo 3 3000.. Ran ultra hle like shit.. But when I upgraded the cpu to an athlon gig, ultra hle ran freaken fast and awesome.. Same thing with epsxe.. Ran like shit on my amd k6-2 533 mhz but fast and good even with the voodoo 3 on my tbird gig..
A good graphics card does help too, but the cpu is more important for emulation.. The graphics card isn't the one doing all the processing for the emulation..
There are people in here still using voodoo 2's but they have a pretty good processor which makes epsxe run fast..
get rid of that p3 500. ouch thats painfully slow. GF3's are not fully optimised unless you are going WELL over a ghz. If you built your pc and your motherboard accepts cu-mine p3's then upgrade to a p3 1000E. They run fast and their pretty cheap these days ($150).
I would have to go w/CPU and memory. I know that you did not mention memory, but it does make a big difference in emulation speed and processing. Ultrahle requires a certain amount of memory to function correctly. Without it it won't run well at all.
The processor is also quite important because many emus, like ePSXe, state that the emulator runs best on a higher end machine.
This, of course, means the processor. So, those are my reasons for saying processor and memory.