Well, this maybe belongs in the hardware forum, but here goes anyway:
iPod Touch devices update once a year, same as the iPhone. Usually late summer or fall (not long after the typical summer release of the new iPhone). They are basically an iPhone without the phone hardware, and almost the same in every meaningful way. The camera’s a lot worse, but they’ve caught up to the iPhone for most other things like including a microphone, and the gryo, and so on.
iPads are released on a yearly cycle, it would seem. Usually in the spring. For games, they’re a great choice - they can run all the iPhone/Touch software, but have exclusive software as well. A lot of games, even iPhone games, play more naturally and easily on the large screen.
Each new version often brings significant changes. New processors that are actually a lot more powerful (the latest is the A5, only available in iPad 2 but surely coming to the next iPhone and Touch later this year - it’s graphics performance is easily 8x the previous processor). Higher-res screens. New internal hardware like a gyroscope. More RAM. Whatever it is, it’s not often that a new yearly release is just a tiny little minor bump over the previous year. Sometimes all the good stuff doesn’t happen until later when the next version of iOS ships.
Old versions: I wouldn’t buy last-year’s model of iStuff. There’s no telling if you’ll get all the nifty features of the next upcoming iOS release, and the hardware usually significantly worse. It’s not “I bought an Xbox after the 360 was released” bad, but there are almost always some serious changes.
What’s coming in new versions - nobody knows, and if they tell you they know, don’t trust them. The rumor mills go fucking nuts all year long with what is coming in the next iPhone, the next iPad, whatever. They’re totally hit and miss. Until Apple announces it (at which point it will be on their site and all over every site on the internet), don’t believe anything.
I don’t think I’d buy an iTouch or iPhone right now. iPhone 5 is due in probably 2-4 months, same with the Touch. I’d wait for that. It’s safe to buy an iPad 2 - it has the hot new A5 processor, will almost certainly get a bunch of new features when iOS 5 rolls out later this year, and won’t be replaced by iPad 3 until next spring (don’t believe those “iPad 3 in the fall” rumors).