What game or games do you think are the most demanding?

I'm in the search for a new game to play since I beat most of the ones I have.
Been playing a lot of demo's.
I must say I've played demo's for FEAR, HL2, Prey and Far Cry. Far Cry was the most demanding game I've played.
everything had to be set on medium or I got low FPS.
I could pretty much max any of the other games and get between 60-120 FPS, except Prey which is locked at 60 FPS, but still got the max out of it.
FEAR had it's moments where it would fall in frame rates but I think with far cry I'll have to upgrade..

Athlon 64 3200+
X1900XT 512
1 Gig of OCZ plat memory revised 2-2-2-5
580 watt Hyper PSU

Oblivion with a bunch of characters fighting at once on the screen is the only thing that has slowed down for me so far and maybe COH on high settings.

I had no problems with oblivion, but I didn't max it out, I let it read my hardware, then I fine tuned it a bit..
I'd love to get another Gig of my memory, but they've discontinued it.
not sure if I could mix two different memory brands..

Obviously you are into shooters so if you haven't played it try the Call of Duty series.Great single or multiplayer action and not too taxing on the hardware.Medal of Honor series is OK also.

Gothic 3, TR Legend and Armed Assault are possibly 3 of the most h/w demanding games I've played... some could argue though, that this is probably down to poor optimisation..

Medieval 2, Oblivion, SupCom (more CPU intensive I think, but can cause a bit of a slow down with lots of units), and Company of Heros are probably the games that are the most taxing on my rig when played at high settings