It really depends what you want to do with the amiga if its just the same reason as me,games,apps, demo's and just messing about with then an A1200 will be fine. The A1200 also offers you the AGA chipset which will allow you to play aga games that you can't do on the A500+. And you can pretty much do the same as you can with a A4000 in expansions but these all cost afew quid to do (GFX cards,sound cards,usb etc etc etc)
but anyway welcome to a.org and welcome back to the money pit :-D