It is an older machine now, about 5 years old, that I bought for work (audio production), but I was wondering if I could mod it a bit and use it for Skyrim.

It is a Mac Pro, Quad core Xeon @ 2.66, 4 gigs of RAM (I would obviously upgrade that), and has a few open hard drive bays. It only has an x1900xt card, which will not cut it.

So I was wondering, could I put in another HD, install Windows 7 on it, upgrade the RAM, and the video card (suggestions) and use it to play Skyrim?

See http://www.gamesas.com/topic/1382904-unofficial-will-my-pc-run-skyrim-thread-60-w-hardware-guide/

I think my situation is something that requires a little more than a browse through a hardware specs guide, but thanks anyway.

Does anyone have any knowledge about this?

Actually, that's all you need. With it being an Intel Mac, you just use Bootcamp or similar to run Windows. After that, it comes down to the hardware.