The 4x5 Graphire is fine for casual Photoshop work. It doesn't have nearly the same level of pressure sensitivity as the Intuos line, but it is decent. If you work at a high resolution though, like 1600x1200, the Graphire is going to take some getting used to. Even the smallest movements on the tablet will be huge on screen. IMHO, the 4x5 work area is best suited for resolutions no higher than 1024x768. Beyond that, 6x8 is the most comfortable.

As for the mouse, you can hook up as many input devices as you have available USB ports. You can have both the tablet and your usual mouse connected and switch between the two as you're working. That's what I usually do.

One thing you might consider doing is trying to track down a used model of the 6x8 Intuos tablet (not the Intuos2). I've been using a 6x8 Intuos tablet for a few years now, and it blows away even the current Graphire tablets. More space to work and more pressure sensitivity. You can probably find them for pretty cheap on eBay.