To the OP
I am running a pretty ancient mobo, with the best gear I can probably get for an outdated architecture. Ive got an AGP machine (rather than the newer PCI-e) with an Athlon X2 processor, plus an ATI HD 3850 512 Mb graphics card. It can do crysis warhead, Fallout 3, and all my games at full settings, but I dont have vista or windows 7 so never seen the DX 10 effects in Crysis 1.
It can run every game I have at full settings, all except on some games dynamic shadows need to be set at medium or it chugs a little. I get best consistent frame rate at 1024 x 768 resolution. As most new games tend to be developed with a dual console / PC route in mind, current graphics demands dont tend to push my machine that far. However there are some 'PC developed' games like Metro 2033 that are pushing the envelope, and pretty soon you will need one of the NVidia GTX range and at least an Intel i5 or i7, or one of the AMD Phenom or quad core chips.
Dual core with a 256 Mb graphics card seems to be the minimum at the mo.
I recently saw a Dual Graphics card (Not SLI or crossfire, it was two GPU's on one board) in the GTX 480 range for £900.00. That made my eyes water ....