Unless you are going to do serious 3D modeling or something... you could get by on a pretty cheap machine.
My mother "learned" illustrator\photoshop on a 200 mhz Pentium Pro.
She is a very patient person though
You don't really need raw processor power for 2D drawing, what you will need is a lot of RAM. Especially if you plan to run multiple instances of Photoshop.
...and a fast and reliable harddrive, (or two for backup purposes) and definitley a CD burner.
The problem is that this is the exact opposite setup that most "cheapie" PC's are now a days. They like to shove the latest procesor into a machine with as little memory and peripherals as they can get away with.
Graphics Tablets are not cheap either. If you get a large one you may find yourself spending nearly as much on that as the PC itself. So keep that in mind if you think you will need one.