Graphics card compatability/upgrading issue

[Tengiell]

Dear Forum

I'm currently considering upgrading my PC's graphics unit to either:

1)GeForce GTX 560 TI-1GB GDDR5-DVI-HDMI-ASUS DX11 TOP or
2)GeForce GTX 560-1GB GDDR5-DCII-HDMI-ASUS DX11 OC

The question is, whether my other hardware is compatible/sufficient for either of those above? The main goal is to get as much graphics value for the money (and most graphics possible when playing Napoleon/Shogun 2 TW) without having to upgrade my motherboard and CPU also. My current configuration is:

ASUS P5N32-E-PLUS SLI Series motherboard
Intel Core2Duo-E6600 2,40 GHz CPU
LC 550W PSU
(Antec 900 cabinet)
64-bit Windows 7 OS
7GB RAM

The store recommends I at least buy a 800W PSU, though different threads on various forums claim 550W will suffice. Regarding the motherboards, they are not sure. Should I upgrade the whole lot or just the graphics card - and maybe also the PSU?

Best regards
Tengiell