Your rig is fine, I would say that the only thing you need is a graphics card upgrade, and possibly a cpu upgrade.
I would say your cpu is still pretty good, but since money is no object
, according to the asus website the only significant cpu you can upgrade to is the -
Core 2 Quad Q9450 (2.66GHz,1333FSB,L2:12MB,rev.C1,4 cores)
Core 2 Extreme X6800 (2.93GHz,1066FSB,L2:4MB,rev.B2)
You might also need to do a bios update.
In regard to upgrading your gfx card, I need to know a couple of details-
What psu do you have, and what resolution do you play at? And how much ram do you have?