Your monitor would be holding it back at 75 Hz but it should be OK, My guess is that it's an older 15" version, most 17" monitors will manage 110 Hz or higher depending on the quality of it, any less than 75 and the eyes will suffer, 85 would be a base Hz for easy viewing for most people.
A 64Mb MX400 will run at 85 Hz NP.
The ATI based 9600 Pro is designated the All in wonder 9600 Pro, there is also a version simply called a 9600, my bad.
Your Nvidia based cards start at the LE, Pro and XLT versions, generally price shows the quality of whatever card, these days of 5 disk games that require a minimum of a FX-5700 to work at a half decent viewing, but a 5200 will run the same game allthough there would be a considerable loss in picture quality and jerking.
Unfortunately once you have used a top end 9800 256 Mb card the price fades into "worth every cent class"
Running the 8X card at 4X is somewhat pointless in reality when most any board made now days all have 8X capabilities, the smaller versions are worth a look if cash is a problem, as they have many onboard goodies so the lack of 6 PCI slots is not a problem, most have the exact same N/B and S/B chips as do the larger full ATX versions, plus they also take a 3200 with no messing about with changing BIOS versions.
Changing memory settings in order to get better frame rates can / is a time consuming job, write down EVERY setting and it's benchmark test and you will find a sweet setting that will give more than the others, on one board I tried every setting possible, oddly enough the fastest was the stock default memory setting, so never assume that changing the settings will increase the speed, it may not, but try it anyway.
Running the 9600 at 4X and possibly having settings set at high will have the opposite effect your after, I have several 9600 cards, but none run on a 4X max board, upgrade time I'm afraid, Even cheaper boards like A7V8X-MX or a A7V600-X and even the A7V400-MX are not expensive @ around $50 USD all are 8X and all should accept a 400mhz -XP3200 cpu, all being backwards compatible with memory to 266 speed DIMM's, so long as your PSU is a good quality one around 350 - 400w, it's too easy.
David.