It sounds rather obvious, but the better specified computer you have, the bettr GP4 will run.
Before GP4 came out, I ran GP3 on a self-built PC with an AMD K6-2 500Mhz processor, 512Meg of RAM and a GeForce 2 graphics card. GP3 ran well on this PC, but when GP4 came out, I found it just wouldn't run except at very low frame rates and in low resolution with most of the display options (mirrors, video walls, etc) switched off.
So I built a new PC, using an Intel Pentium 3 1GigHz processor, 768 Meg of RAM (PC100) and the same GeForce 2 graphics card. It ran well at high frame rates, but the processor occupancy was frequently very high and occasionally, when cornering amongst a pack of other cars, the game would slow down and then pick up again. I couldn't run high-res cars with it without sacrificing the frame rate.
That PC developed a motherboard fault after a couple of years of heavy use. So I had to build a new one in a hurry. The next PC, which I still use now, has an AMD Athlon 2GigHz Processor, a Gainward GeForce 4 graphics card and 512Meg of PC133 RAM.
The detail from the Gainward graphics is superb and the game ran very fast with all screen objects, except for heat haze, switched on. Until this week, I ran the standard cars with only one modification - Viper's 2003 Ferrari. However, this week, I've downloaded Steven's 2004 carset, helmets and pitcrews, all at high res and the game now looks fantastic, but slows down frame rates in occasional corners in the pack again.
I'm going to add more RAM to my PC in case this will help to speed it back up. But maybe after Christmas, I'll be building another PC and make it faster still.
I think the OS also has an effect. I have a friend who runs GP4 on a similarly equipped Windows XP computer and GP4 doesn't seem to run as smoothly as it does on my Windows 98SE. That's the exact reason why I don't want to upgrade my OS, because GP4 seems to run best for me on Win98SE. Has anyone else found this to be true?
Paul