Tablet PC's aren't that good. The pressure sensitivity is far lower than the tablets itself.
Unless you're going to get just the Tablet with a built on screen, Wacom Cintiq which then is a far superior tablet but much costly.
Intuos is better than Graphire but graphires are a lot cheaper. I have a Graphire3 and the one in the market right now is a Graphire4. Slightly better than 3 and price is reasonable. But if I had to choose, I would get an Intuos. Size? 6x8 Will work fine for me. (4x5 is the size I have and it's quite small for what I need to do)