Your 3d mark scores seem a little low for a 7900 GT. You running the latest video drivers, motherboard drivers, sound drivers, etc?
(you may not think motherboard drivers are important to game performance, but they truely are).
I myself have one (which is UNDERCLOCKED) and I score 8k in 3D Mark05, and around 6k in 06.
AMD 64 X2 4200+ @ 2.2ghz
2048mb Corsair PC3200 RAM (2x1gb)
WinFast NF4SK8AA Motherboard
660GB HDD Capacity (3xHDD)
600w Enermax PSU
eVga 8800 GTS 320mb