Pentium 4 processors aren't hugely expensive anymore. You may be able to find a 1.8-2.0 GHz for around $150 - 200 if you look around for it. You should see a bigger increase in speed. Trust me, the 9200 is a big difference over the MX, and it's the processor that's bottlenecking you. You might want to go out and find a Pro version if possible, but they're hard to find. Online shops may be your only option.

As for overclocking, if you have good airflow, you can probably get it up a good 5-10% with the default cooling, but a new fan might let you get up to 15-20% faster, which will show a big increase, but will probably shorten the card's life span even more than a normal overclock.