New graphics card for my Mac Pro (early 2009)

[dulnan]

As my Mac Pro just turned 3 years old a few days ago, I planed to upgrade a few components. I've already added 16GB to the default 8GB RAM it had and bought a new SSD.

Now I also want to get a new graphics card, but I have no idea which one to buy. I use my Mac mostly for video editing in FCPX, animation stuff in After Effects and fooling around with Photoshop and Aperture.
I was happy with the overall performance of my Mac until I updated to Lion. Since then, UI animation (like switching spaces in Mission Control) is very laggy, probably because I work with two 24" Dell monitors.

Here are some specs about my Mac Pro:
- 8-core Mac Pro (Early 2009)
- 2 x 2.66 GHz Quad Core Intel Xeon
- 24GB RAM
- ATI Radeon HD 4870


I have looked through a few posts here and I'm just more confused than before. Here are the possible options I think I have:
- buy one ATI Radeon 5870
- buy two ATI Radeon 5770
- keep the 4870 and add a 5770
- keep the 4870 and buy an used NVIDIA GeForce GT 120
- buy random PC graphics card and flash it
- stick with what I got and wait for some new graphics cards to arrive from Apple


What would you recommend? Thanks for any advice.