It's a nvidia 8400 gs. so the drivers should still be available. Also it's a half hieght card so pc form factor shouldn't be a problem. Also being pci based it shouldn't draw enough power to stress out the PSU. Plus it's practically guaranteed to work in your PC.
If you have any intel macs, make a seperate partition on one of your drives and install windows as a dual boot option on one of your drives using bootcamp. Most of the newer intel macs are coming with at least half decent graphics chips.
Thanks to everyone who responded.The drivers on the PC are all up to date according to RadaraSync (http://www.radarsync.com). I used to have Windows installed on a Mac but I prefer the two-screen system and it was working great until I "upgraded" the monitor. So I'm looking for the least expensive video card that will support the 1440 x 900 resolution.