Crysis and Metro 2033, with combined Radeon and GeForce

I need some advise for upgrading my system to get full HD (1920X1080) when playing Crysis at Metro 2033. Mine sort of:

- AMD Phenom II 1055T
- Team XDark 12800 4X2Gb
- WD Raptor 300Gb (main drive)
- Radeon HD5770 in CF
- Cooler Master SP 600W

With Crysis (19x10, DX10, enthusiast, 4AA) my system only managed to get 19-21fps, and Metro 2033 (19x10, DX11, all maxed out, 4AA) my system give up at 15-16fps.

I've ever read an article here about combining out Radeon (as main display) and GeForce (dedicated Physix). My experience with Radeon compare GeForce, it has slight darker shadow and less realistic effect. Those article I've read mention that a system with Radeon can play game with more realistic effect (Physisx) that GeForce had and can push the fps about 150%. Is it true and anyone had experience with it?

I've already planned to get HD6870 (cheaper, single card, more powerful than mine), but don't know what GeForce card needed to be added to makes those games run at playable fps (maybe at least 27) with Physix effect but also with affordable price (max $250-300, my plan is GTX 460, but still need advise). Anyone could help me?