Poor 4870 X2 performance w/ my new 30 inch monitor

After doing a fresh OS install (vista 64 bit), I am getting poor performance with my 4870 X2 and my new Dell 3007wfp 30 inch monitor. Before my new Dell monitor, I was running a 24 inch Samsung with the 4870 X2 and everything seemed fine. Of course I expect to see a performance drop due to the massive 2560 by 1600 resolution but my FPS in games is no where near some of the benchmarks done on websites. For example, in L4D with everything maxed out and AA at 8x, it is completely unplayable (under 10 FPS) while benchmarks show with the same settings that they are getting around 60 FPS. Besides L4D, I am experiencing the same results with FEAR 2. This also brings up the question whether the fact that both of these games are ran through STEAM has anything to do with this. Anyways, after running 3DMark Vantage on Performance, I am getting around 14,300 on the GPU test which is completely normal for a 4870 X2. I am just wondering if anyone might have any sugestions for me as I know that not too many people use 30 inch monitors.

Here are my system specs:

CPU: Q9550 OC'd @ 3.6
Ram: 4 gigs at 900mhz
Power supply: HX1000w
Board: EVGA 780 FTW
Card: Sapphire 4870 X2 (Ive tried several drivers now including the newest 9.3's back to 8.12's)

Please if anyone has any ideas or questions let me know. Thank you!

Yes, all i did do was change the monitor but also did a fresh OS install (something i like to do every few months). I havent tried running it at a lower resolution yet, i gues i will now. When I turn off AA, it becomes playable (60 to 70 FPS) but why am i getting such horrible performance while the benchmarks showing otherwise?

I've noticed this sort of behavior when I reinstall windows and forget to install DirectX. That causes graphics to be slow and glichy. Another set of drivers that has indirectly caused issues are the chipset/motherboard drivers, so make sure those are installed and up-to-date.

are you positive the card is in crossfire mode