Ipad 2, which shares the same hardware as the brand new Mini, will be supported for years to come. The A5 is still blazingly fast on something as optimised as ios. The ram will actually hit you much sooner, but that will still be another 2-3 years away for most apps, and as long as you don't constantly have too many open tabs in Safari.
Whatever you do, do not get an ipad 3. Nothing wrong with the device, it's just pointless if you don't really value the retina. This is especially true if you use your ipad predominately in landscape mode. Really, the only practical thing retina is going to give you is make potrait mode useable (most things are a joke in potrait mode on the 1024x768 iPads). Not only that, but you are getting a heavier, thicker, less nimble device than the ipad 2, with no advantage other than retina.
Having said all the that, I can't recommend upgrading to the ipad 4 enough. Not only do you get that retina display (usable potrait mode), but the A6X is really a world apart from anything else. The difference really becomes noticeable over time. The lightning connector is also a huge improvement over the old clunky 30 pin connectors (again something that really becomes noticble over time).
Forget about the ipad 5 too. It will be far more fragile if it does become part of the iphone 5/ipad mini design family (completely pointless for a device as big as the full sized ipad imo) And the hardware (cpu, ram) probably wont be significantly better. The huge improvement will probably be the weight and thickness, which is something i dont really value anyway because i dont really hold my ipad when using it.