I think you need to go back to the power supply issue. I think Roo's wrong on this one. I think it *IS* very possible that your PSU is 150W. My 5-year old machine had a 175W PSU, my sister's has a 150W, and it's only 3 years old! Companies like Dell, HP, and Gateway get away with REALLY cheap PSUs.
There should be a sticker with lots of numbers on it. There should be a listing of "max continuous output" or "total output" something like that. It will be the highest number on the list. Example: My current one says 450W (-ish).
If you really DO have a 150W (very likely, with such an old case) then you're not going to get anything from any modern video card -- you just won't have the juice to run them at full power. It would be like underclocking them, all the time.
EDIT: Go back to the sticker. This is one example. Often each manufacturer will have their own layout for displaying this. In this case (pulled randomly off the web) I have circled the max power listed.
