For what sort of footage- DV, DVCPROHD, HDV? All of the current Mac Pros do great with those formats. You just need plenty of hard drive space, latest version of FCS and at least 2GB of RAM.
I recently added a new video card to mine. Since ATI released the X1900 mac edition. That doesn't really affect renders etc. If you are dead set on upgrading I would go with ram. It's the fastest cheapest upgrade you can do. I run 6g ram in mine.
So, the only thing that really affects render time is processor and RAM, then? I'll upgrade the RAM to at least 6 GB - how much faster do you think that will make things?