Upgrade your CPU and it'll abosolutely fly. Even entry level CPUs are great performers.
If your CPU is less than 1ghz there's little point putting a Rad8500 or GF3/4TI as the card won't reach any where near it's potential. Put simply your Rad is waiting around for your CPU (and poss mobo) to throw the data at it.
Under 1ghz you hardly see any gain going from a GF2TI / Rad7500 to the top cards, however when your next upgrade is due a faster CPU is all that's needed.
Finally check things such as your RAM (256MB min esp WinXP), drivers (mobo etc, not just Rad), O/S (eg. Win95 makes crap use of latest hw and extra RAM etc), bus speeds and stability (esp when o/c). A lot of people like to push their FSB up to 125 from 100 but it kills the PCI and AGP speeds as they run below 33mhz & 66mhz (at 1/4) until 133mhz bus. With 4xAGP that means it'll be running at 250mhz and not 266 (if left at 100 or 133 bus). The most you can usually get from the PCI is 36mhz (stability wise).
Also try using (or forcing) AA and AntrFilt etc as this is work done almost soley by the gfx card and may help the CPU limitation out a bit.
Hope that helps