I have 2x that RAM. Muahahaha.
I'm just fine with my hmm $100 one. The bigger surface area lets you do more fine details in whatever program you're working with, but don't forget that photoshop lets you zoom in. So the $100 one might just be fine.
Also, don't forget that Wacom sells refurbished
6x8 (the big size) tablets for only $150. They have a full year warranty on 'em, too. That means that if it does crap out, they'll hook you up. It seems like a pretty good deal to me.