Upgrade or new mac?

I'm looking to make the jump from FCP 6 to FCPX. I currently have a Mac pro 3,1 (2008) with 2 quad-core 2.8ghz proc. and 4GB of ram and an ATI Radeon HD 2600XT.

I know I need to bump up the ram and change out the video card to get FCPX working on the system. My questions are:
What video card should I be looking for?
Do you think FCPX will perform well on this cpu?
Is it worth buying the iMac to replace the entire system?

Exact same Mac with 8GB RAM (you'll want at least that much for 64bit OS and applications) and Radeon 5770 which you'll need. These are relatively inexpensive upgrades... but FCPX seems to scream on a Quad i7 and a MacPro replacement is fast approaching. You can certainly make do with MacPro 3.1 by upgrading the GPU and the RAM though.

I have an "Early 2008" Mac Pro. I've got 16GB RAM and the Radeon HD 5870. It outperforms a brand new, maxed out 15" rMBP working with native REDCODE RAW 4K files.

I'm holding off until the new Mac Pros come out. And as soon as OS X 10.8.3 is released, I'm going with a Sapphire Radeon HD 7950. 10.8.3 beta already has native drivers for both it and the NVIDIA Quadro K5000. Although the 7950 on paper looks like a better FCP X card. I've communicated with someone at Barefeats who has first hand knowledge, and says the 7950 will run in my Mac, and give it a huge performance boost. They can't release benchmark test until 10.8.3 is made public by Apple, obviously.

I also figure I can take the 7950 to a new Mac Pro later this year when they're released. RAM won't transfer, because there is newer, faster RAM now. But that graphic card will transfer over.

MacRumors.com has articles about the 7950 , K5000 , and an Apple branded SSD for the new Mac Pros.

Thanks for the input.