Should I buy a new graphics card?

I entered into a "more detailed than necessary" examination of this. The basic questions are:

• What will it cost?
• What sort of improvement to performance can I expect (can I play Shogun 2)?
• Will any power savings pay for some new card?

Given my current rig is a pair of nVidia 8800 GT (2 x 512MB) in an SLI config; and Shogun 2 reports graphics requirements as "AMD Radeon HD 5000 and 6000 series graphics cards or equivalent" (pcgamer.com), I had no idea. I went looking for benchmarks, and couldn't find everything (my old/current  rig, and some equivalent to the Radeons) on the same page.

I compiled some stats from previous years, did some (very) rough math and came up with this worksheet:
GPU decision worksheet. It turns out that my pair of 8800GT offers identical performance to a (more) modern GeForce GTX 460 (1GB RAM), which offers sufficient performance to play Shogun 2.

The upshot of this? I can play Shogun 2 on my current rig, and it's very unlikely that I can recover the cost of the new GPU through power savings. No new kit for me :-(