715 is right on the money:
At the time of its release, the 7600GT was the best AGP card out there, beating the 7800GS and using less electricity to do it. It is a significant upgrade over the ATI 9600 and 9800 AGP cards that a lot of people were replacing with it.
Games other than AH, such as CounterStrike: Source, saw a significant framerate increase by swapping the older cards I mentioned with the 7600GT. In AH, however, in these 4 or 5 year old machines the CPU is the limiting factor.
Jacko, at this point, if you are trying to increase the performance of AH on your rig, then more CPU speed is the way to go. Overclocking will get you small gains - figure a 10% overclock will get you about 10% more framerate. Getting a faster CPU that still fits into your vintage motherboard can help so, but only to a degree. If you can find an Athlon at 3 GHz that fits your motherboard (and I'm afraid I just don't know the AMD chips of that vintage anymore), then your framerates should improve about 50% over what you have. If I were to guess, you have an "Athlon XP" with a PR Rating of +2000, and the highest that generation of chips went was +2800, which I see goes for $68 on Pricewatch.com's list of vendors. That would translate into a 40% framerate increase, which means if you current get 30 FPS on average, the upgrade would get you 42 FPS on average.
The better videocard should let you increase the Antialiasing levels without hurting framerate, which smooths the "jaggies." Therefore, you could decrease your resolution (from 1280x1024 to 1024x768, for example) which increases your CPU-limited framerate, and then increase antialiasing levels to get rid of the resulting "jaggies." You can (and should) also reduce the texture size and play with the visial detailing sliders to increase framerate too.
So while you can't expect miracles, you can get some improvements if your system settings aren't already optimal.
-Llama