Exactly what video card are you running, and what drivers version are you using? What resolution and color depth are you running AH at?
A PII 400 is going to have some troubles with AH though. That's on the low side for computing power, and AH relies heavily on Floating Point math from the CPU to run fast. I think the advice you got was likely good... your best upgrade will probably be the CPU, but in order to do that you are going to have to do a major upgrade... Mainboard, CPU, RAM, possibly video too.