I did the same with my 8800GTS 640mb for the same reasons. I had 3 profiles that I mainly used, an underclocked configuration that loaded at startup and I use working in office apps & general stuff. Then I had my max stable oc config that I loaded when I wanted to play a game. I also had the default/stock settings, that I used when doing some 3d work. It worked great, and I plan on doing the same with my 512mb GTS - I just got it back from my stepup eariler this week and now I got a new cpu & mobo coming so the underclock is on hold til the new setup (Of course I already OC'd it). I did the same with my x1950xt that I had previous to the 640mb as well, and even was able to undervolt that one with ATItool.
I used RivaTuner for this, and I just setup hotkeys to switch the clocks, worked great. I set it up so ctrl+pgup is the OC'd config, ctrl+pgdown is the underclocked config, and ctrl+home as the factory/stock clocks. Made it so easy to switch between clocks, and never had problems switchin em even if I forgot to do it before starting the game. I also use hotkeys to turn on & off Rivatuners OSD, and have it show core/shader/mem clocks, gpu temp, fan duty%, vram usage, and finally FPS. That way I can verify my settings while in game, & monitor temps, fan, FPS in every & any game. Heck, the OSD will even work in window mode when I work with 3d stuff and will show FPS. It works out great and have had absolutly 0 problems using this kind of setup over the past 4 months or so I've been back in the nvidia camp. When I first switched back to the 8800 from the x1950 I was missing stuff like this that was pretty easy to setup with ATItool. Once I read up a little on Rivatuner though, I like it even more.
As far as going too far underclocking, I don't think there would be much danger there. I actually dropped mine down quite a bit without probs. The x1950 I undervolted a bit too much when initially setting it up and it just gave artifacts at the desktop lol, just bumped the voltage back up and I never experienced any problems with it for the next year that I had it. Going too low shouldn't do any harm other than not be stable at those settings ie. you can't fry the card underclocking and/or undervolting. I suppose if you went to extremes maybe it could do something, I dunno. I ran 25% lower than factory clocks with no problems and over time never degraded performance or stability with my max stable OC on either ATI or Nvidia cards.
EDIT: I just noticed this after posting and have to say, damn OrooOroo I've never seen anybody in any forum with that many posts! :shocked: :beer: