I installed a Radeon PCI in my friend's 800 Mhz P3 system. That was almost two years ago. It was too slow then and is horrendous today.
The only advantage the Radeon had for my friend is while his onboard graphics (Intel 810, not 810e) allowed only 4 MB of shared memory which limited 3D graphics to 640x480 and only 16-bit colors the Radeon let him use 800x600 in 32-bit colors. The Radeon did manage 30 fps in Quake 3 at 640x480 and 800x600, high detail. 1024x768 dropped to something like 10 fps, though. 3DMark2000 scored 3600 points. 3DMark2001 score was 1500, I think. I only got it to complete the test once with those old Radeon drivers.
At the time, 2 years ago, the Radeon PCI (128-bit SDR memory) was faster than the alternatives, Geforce2 MX PCI and Voodoo4 PCI (3DFX was already defunct at the time and I didn't know there was a TNT2 PCI).
The Geforce2 MX PCI and Voodoo4 PCI were more expensive than the Radeon PCI. The Geforce2 MX had horrible visual quality. No support from 3DFX plus the VooDoo4 was power hungry and ran very hot which wouldn't be very good in a MicroATX case so I recommended the Radeon PCI for my friend.
He's a bit of nit-picker. I once updated his 810 chipset drivers which changed the default gamma on for the desktop. He phoned me saying something like, "This video card is crap! The colors are all messed! Everything looks different! The icons and text are too small! I want you to take out the new video card so that I can return it!" I looked at and adjusted monitor contrast and changed desktop resolution to 800x600. Problem solved. He also complained the noise from the Radeon made his system much louder. Being an overclocker, I couldn't even hear the fan, but I noticed he had moved the computer from the floor to the desk. When we moved it back to the floor the sound was much better (to his ears. I never heard any sounds).
As fussy as he is though, he never complained about the performance of the Radeon PCI. He got to keep his MicroATX OEM system and he could play his NFS5, the only game he plays. Everything is relative. If you have a crappy system any gains help I guess.
Incidentally, my niece has a Celeron 667 Mhz system with improved Intel 810e graphics (11 MB shared memory). I never really had a chance to benchmark it but it seemed faster than my friend's original configuration, at least in games. It also has full 32-bit color in 3D. I don't think there would be enough of a gain (if any) switching to a PCI video card in her case.
So there's my lengthy opinion on the subject.
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