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

Thats not a bad rig you have.Another gig of ram might help.I ran Farcry on a 1.8 ghz athlon xp and FX5200 and it ran fine though not at maxed settings so you should be able to.My current 4000+ and 7900gt have no problem with anything so far.Your 1900 is comparable to my 7900 .You might even have a little edge on me.8549 on 3Dmark5 last I checked.

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.

the only games i have experienced a drop below 40 FPS is Medieval 2 and STALKER.

I found with medieval 2 the army size can greatly change the FPS, on my rig i was running a 6 player castle siege with Huge armies and the frames drooped to 24. Also i find STALKER has its low FPS moments especially when in high HDR light areas, however i do believe there are many bugs in STALKER cause in opinion they released the game way to early and now they are paying for it in bad reviews. These two games have been the only drag but i am expecting that crysis and Age of Conan will test my rig's extremes.

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..

So what was the question again?

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.

Fear is more demanding than Far Cry, also you can mix brands of memory, just make sure they are the same spec.

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..

I have an X800 and the only game I cant play at high settings at 1680X1050 now is FEAR... Armed Assault ran very well at native 1680X1050 with all the settings at high bar the shadows.

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