I've done it - it's best to do it while you're swapping a hard drive, as you're right there anyway.
It's not at all difficult, but finding the kit can be - I think that Apple got wind of what people were doing, and put out a note that the kit for the Mac Pro should only be installed by an Apple authorized service person, or something to that effect.
I found mine with one of the big Mac 3rd party vendors in Oregon, and paid less than $50 for it.
You do need the 802.11n enabler, but it should come with the kit.
Quite honestly, I don't see a significant difference in performance from the original B/G card, but it's nice to know I do get better speed and range - I did notice a difference in the range I got with it.
If you have any questions let me know...
MacDann