quite like you want it to.
You will be able to get a HD of the required size, inside a Firewire/USB 2.0 enclosure, which will connect to either the PC or the Mac.
Quite how you plan to connect it to the WIreless Router, or why, I don't know. You may be thinking a step too far on this one.
A drive for the PC and/or the Mac is doable.
Is it your intention to have a NAS(Network Attached Storage) device? If that is the case, your price just went up and it will not connect directly to the Mac or the PC, just to the ethernet cable (not wireless)
I run an Apple PowerBook G4 Tiger 10.4.3 with two USB 2.0, one Firewire400, ethernet, wireless, so on. I also run a windows laptop. I need an external hard drive that can plug into the wireless router (Linksys Wireless G), the USB on the windows laptop, and the Firewire400 on the PowerBook. I need at-least 250GB, or a Hard Drive case, in which I can mount a hard drive, that I buy separately.
Please help me out.