Personally I wouldn't go the ipad route when there are so many tablets around that you can nab that will offer so much more. The stylus options are pretty bad and you won't get the same pressure sensitivity as you would with a wacom digitizer. What I ended up doing was picking up a refurbished hp elitebook 2730p for around $200-300, and threw in some extra ram, and bam. Full laptop with wacom support that you can fold into a tablet or use like a regular laptop and run SAI, Photoshop, any and every program you're used to. With the ipad you're restricted in what apps you can use, how much space you have and how easily you can transfer to your main computer.
Honestly the processor has been managing pretty darn well on the thing since it's a full powered dual core so still manages better than some of the under powered atoms you'll find. SAI painting and inking up to 350 dpi is smooth, and Photoshop's been doing well as long as I stay away from the 500 pixel brushes, which does cause some minor lag. The fact that you can stick a SSD and 8 gigs of ram in it means that despite it's age, you still get a machine that's beefier than a lot of the slate form tablets. You really can't do much better for the price range. If you can afford a little more, I've heard good things about the asus e121 but I think those are still kicking in the 500-600 range.
If money isn't a real issue, you have the new samsungs, and if Microsoft gets tired of playing chicken with adobe over wintab, the Surface is a *beast* of a beautiful machine for 1k. For a little more than that, Levono Helix is looking rather insane but it's not out yet.
No, you need a computer for a wacom tablet. The Tablet is just like a fancy mouse.
If you want to draw anywhere on paper and then upload it later you'll want something like this: [link]
Or you can get a tablet with a stylus. Microsoft, Samsung, Apple, and a fuckton of other people make tablets that would work. Some come with styluses and some (namely the iPad) require 3rd party styluses.