Tested:
Creative Blaster GeForce 256 DDR 32mb AGP 4x
PNY Geforce4 MX420 64mb PCI

I have these two cards and as I only have one monitor here in uni I decided to take one out of my system to reduce temps and extend life of a card.

I tested using 3dMark01 SE at 1024x768 on an Athlon Barton 2500+ @ 3000+ (10.5x200) , Asus A78NX-X with 512mb PC3200 CAS2.5, Det 53 drivers

Geforce 256 DDR (AGP 4x):
Default (120/300 ): 3564
140/340 : 4110
150/350 : 4287
160/360 : 4492

Geforce 4 MX420:
Default (250/338 ): 3515
275/400 : 3818

I stopped testing the GF4 at this point as it was clear that it would not be able to catch the AGP card, being limited by the bandwidth of the PCI bus.
The only test where the GF4 was significantly better was the High Polygon Count tests, especially with 8 lights. Presumably this is due to the better T&L on the GF4 core.

The first test I ran on the default GF256 I got 2588 - thought this was a little low, so went to update drivers.. "Error: Your drivers are not compatible with Windows XP and the default drivers are being used" even though they were Det52s! So just using the nVidia drivers boosted me nearly 1000 and the overclock gave me nearly another 1000. Not bad for a graphics card going on 6 generations old :P

The moral of the story is that PCI cards never have enough bandwidth to be able to take on even an inferior AGP card.