I presume the 255.33 drivers are the one from the CD that came with the card. Never use the drivers from the CD. Download the 266.58 drivers and use those. And remember to use Driver Sweeper when you uninstall the old ones.

If youre having fps drop then try the max performance and new drives, but at source engines the gpu usage is generaly low, Im playing portal 2 now and gpu keeps at 20~30%, but there are no fps drop, about the 570 isnt recoganized by steam, well I guess that valve take a little time to update their list of VGAs, remembered when got my 470 was the same thing.

I am having a similar problem with my 560ti. All source games are running at very low gpu usage while my cpu constantly runs at 90+% usage. I had an evga 8800 gs before that maxed out all of my source games with no issues. When using my 8800 my gpu usage would sit at 90+% and my cpu would only be around 60%. I am running at 1440x900 res with 4xaa and I sometimes see fps drops into the 20s. All of my other games run fine(I can play BF:BC2, Crysis, Crysis Warhead, and Metro 2033 all maxed with very good fps)
 
PC specs:
Intel Core 2 Quad Q6600 OC @ 3.4 GHZ
EVGA 780i Mobo
4GB Corsair XMS2 DDR2
Corsair 750 watt PSU
EVGA SuperClocked 560 Ti 1GB
driver: nvidia 275.33

hmmm i play TF2 a lot and i don't have that issues
1. download the new drivers 
2. it could be using you cpu more than your gpu but 60c 18% hmmm that is not right 
3 up date the patch for tf2
 
when i finish uni i see what my computer does 
 
but if the FPS is more than 35 your fine. 

I mostly play left 4 dead 1 and 2 but the problem is apparent in all source games.
Settings: 1440x900 all settings very high-high 4xaa 16xaf
L4D2 running on 8800:
CPU USAGE: 50-60% GPU USAGE: 90-95% FPS:75-60(with vsync)
L4D2 running on 560ti:
CPU USAGE: 95-100% GPU USAGE: 20-30% FPS:drops into the 30's(with and without vsync)
 
35 fps is not fine for my setup since all of my source games ran at 60+fps on my 8800 maxed at the same settings.
I have uninstalled and reinstalled my drivers using driver sweeper around 10 times now, nothing helps.
Nearly every non-Source game I own runs fine with my 560ti. My CPU was more than capable of running source games when paired with my 8800.
For some unknown reason Source games seem to only be using my CPU now and not putting any of the load onto my GPU.

I've not had that problem but with tf2 I have had a different problem since I got my 570
 
I have to set it to compatibilty mode to play it as it crashes but then it would crash in compatibilty mode so I turned it off and now it crashes when not in compatibility mode LOL
 
I think the crashes can be asigned to different drivers LOL
275.33 need compatibility
275.50 not need
280.19 needs
 
I don't care as long as the game works just a strange thing I get with tf2 (l4d/l4d2 or other source games I have are not affected and work fine only tf2 needs compatibility mode???)