Let me guess, GP3 runs a lot quicker then GP4 on your pc? Well thats simple, your getting low frame rates because of your Pentium 2 400Mhz processor, its a bit slow. Sounds like you need an upgrade!!!!!
GP3 will run reasonably quickly in 800x600, with a lot of settings lowered.
GP4 on the other hand does require a lot more powerful PC to run at decent speeds though. But if you look at the GP4 box, the minimum specs are: Pentium 2 400mhz, 64mb RAM. For games to run quickly, you really need a PC thats at least near the recommended specs for that game.
You could download and use the latest Detonator drivers from Nvidia website (unless your usuing them them already that is), make sure you are using the latest version of Direct X (V8.1), you could overclock the graphics card to get extra speed, overclock your CPU for mre speed but seeing that your motherboard is probably old, you might need to change the jumpers manually though, but the good thing is your CPU probably is not locked so you can probably overclock it by modifying the multiplier clock.
But obviously overclock components to a sensible level and overclocking is done at your own risk....