graphics cards

Hi there,
What would roughly be the expected time for rendering if we chose GeForce 8800 ?
And what would be the expected time for rendering if we got a GeForce FX 5600 instead?

We are new to Final Cut, although instructed on how to import a miniDV movie through this program.
All we do is to import from our DVcam, edit a little (about 10 cuts in a 1 hour movie) and export into a Quicktime file.
We mainly export into 3 formats =
- no compression
- DV PAL
- H264
Our experience is amatorial and we are considering using iMovie also.
The big problem is rendering: it takes a long time to be completed.
At the moment we are using an old G5 - 1,6 GHz with little RAM.
Our rendering lasts from 8 to 10 hours (the content of 1 hour mini DV tape)

We are going to buy a Mac Pro, provide it with 8 GB memory (RAM), 2 internal scratch disks (s.ATA) and we were thinking about getting a good Graphic Card.

What would roughly be the expected time for rendering if we chose GeForce 8800 ?
And what would be the expected time for rendering if we got a GeForce FX 5600 instead?

thank you
Albino Gallo