I have one Mac and 2 PCs.

Hmm, I think it's just a matter of using the disc that comes with the router with the Mac (connected via Ethernet) and follow the set-up, and then connect the PCs to it wirelessly or through ethernet

Well, the PCs are connected to the Airport for internet access anyway. Can I access the Mac or PC from the other somehow? Even still, I need a way to have a partition for backing up my Mac's data and a partition for backing up the PC data. transferring between the two is not something I need often and isn't a current concern. I just don't have my own USB Hard Drive yet and as you can tell from my other thread, backing up is essential.

I don't know, I haven't used that router

This may be helpful.
GUIDE: Networking Windows with OS X - Mac Forums
External hard drives are cheap now days, it might be about as easy for you to buy two seperate drives. One with NTFS for Windows and another with HFS for your mac.

Yes, I'm going to get an external enclosure but the HDD I would put in it has not arrived.