Ok. I have an 8800 gts 320 mb, intel 540j 3.2, 2 gig corsair xms 675 mhz ram and a thermaltake 660 watt sli certified power supply.
I can run HL2 and Ep1 @ 1680 x 1050 max settings. Everything.
I can run EP2 the same on all resolutions EXCEPT 1680 x 1050.
When I switch to that resolution, it runs literally at 2-3 fps in the menu. It takes so long to click anything I haven't bothered trying in game.
Any reason as to why it does this? All other 16:10 resolutions run fine in the menu.
Not necessarily sure what the problem is. The Source engine is made with great resolution codings so that performance impacts take place after increasing or decreasing large amounts of screen resolution instead of the very next choice. (Framerate doesn't go so stinking low because you choose a slightly higher resolution, and can go really high without problems).
But if you can run at great FPS all the way up to before 1680x1050, do you necessarily need that exact resolution? In a game there won't be a ton of difference from the resolution before this one, unless it's one of the resolutions that offsets your screen so it looks all weird. Also, the menu tends to run a little slower sometimes than the actual gameplay, I think it is related to the all the menus you can click on, while having a running 3D background behind all of this.
So I recommend you either...
1. Don't run at 1680x1050
2. If you for some odd reason, hate anything under that resolution, and must run at the resolution, then I suggest you try actually running the game.