Hello
I do an experiment.
I unplug my VGA graphic cart (VGA 8X, nVidia FX 5500, 256MB) and plug a PCI graphic cart (Trident3D, 4MB).
Now my pc runs faster (about 20%).
There is no problem with power supply (AcBel 450wat real).

Let’s imagine:

The Chipset in main board (Intel) work like a 3way pipe (1.CPU, 2.RAM, 3.south Bridge)
When all way is open each them have 33.3% traffic, but when one way is close (VGA) other way have 50% traffic.(CPU 50%  ,  Ram 50%)
I think this is the reason of speedup.

What is your idea?
Thanks
Bad logic.
Both PCI and AGP run outside the 3 processes you listed...CPU, RAM, Southbridge.
Although they may independently rely on each other for resources and THROUGHPUT there should be no difference between what you have stated...
Actually it should be the opposite of what you are seeing.
The AGP card should have better system performance than any PCI card.

P.S. Although you trust it the PSU could actually be the CULPRIT here as the AGP card PROBABLY has higher demands than the PCI.