|Depends on the chipset. If the onboard video doesn't support certain API calls like T&L in hardware (Intel GMA950 and earlier), then the CPU would be used to handle T&L calculations. |
However, modern integrated video solutions support all D3D and OpenGL API calls in hardware, so CPU horsepower is no longer a concern.
Forget about PCI video if your system has GMA X3500, GeForce 8200, or Radeon 3100 integrated graphics or newer. A PCI video card may actually slow you down, since modern PCI-Express chipsets aren't as well-tuned for standard PCI performance as the older PCI/AGP chipsets were.
Super PIII - Still Tickin' | New unlocked ES P-III @ 1.8GHz (150x12, 1.65v) | Apollo Pro 266T | 2GB PC2700 | HIS x1950Pro | Swan M10 speakers | 500GB | Win7 Pro