Sorry to throw a spanner in the works, but the short answer is...
Yes, you can.
There are a few plusses to this -
Firstly, you can get the latest graphics cards available quicker than the Mac variants (although that's changing now...).
Secondly, these cards are
cheaper than the Mac cards (and that's not likely to change in the forseeable future).
That's because the Mac cards use a proprietory connector (Apple's own ADC, which run their range of LCD displays), whereas the PC cards will only have VGA, or if you're lucky, a DVI.
PC cards also have a few more options:
Usually, a S-Video output, and a TV-tuner built-in (although, as before, this is starting to change with the Mac as well).
The downside to all of this is that you have to "flash" your graphics card - I don't quite know how to do this myself, but there are a few links from the good people at
http://www.xlr8yourmac.com on how to do just that. It involves loading a small program into the PC whilst it has the graphics card installed, and changing the settings via the software. Then, the card can be re-installed inside a Mac, and - theoretically - it should work (somebody here might be able to give you more of an insight on how this may work).
If you only have a VGA monitor and want to upgrade your card, you can only really benefit from getting a "flashed" PC graphics card. The drivers are ostensibly the same, and you can really pick up a bargain from a second-hand computer store (I saw a GeForce 3 with all the trimmings 2 days ago for £25! The Mac version wouldn't give you much change from £160, if you're lucky enough to FIND one...).
Good luck!