I have an old model Intel Mac Pro model 1,1
Dual CoreXeon core = 8ghz
9 GB of ram (2-2-2-1-1)
Bus speed 1.33 Ghz
NVIDIA GeForce 7300 GT (PCle x16, 256MB)

it can do dual dvi output and im looking at adding a second monitor (one for working, second to show output).Both monitors off the same card.

Looking to add a second card to boost rendering and wondering if there is a big improvement?
using Adobe CS5 and may pick up FCP Later.

Replacing the 7300 with a Geforce GTX 285 should help in CS5 since it will enable CUDA hardware acceleration for the Mercury Playback Engine.

What he said. Or Quadro 4000/4800 if you can afford it for even greater performance boost.