Knowing for what you need or are going to use the device, will help for sure ...
They come in different sizes, where most common or popular are 7 and 10 inches. The 1st one is more portable and easy to carry one. The 10 inches screen gives you a better experience for videos, magazines, PDFs and even browsing.
Even though Honeycomb is the most recent Android version and all people are talking about, Froyo can meet your expectations and the tablet will work perfectly fine. A good example of that is the Galaxy Tab, which you can get in USA for about 500 dollars.
Some people use tablets as dedicated e-readers, other as multipurpose device or even for games. Like I said at the beginning of the post, knowing what you want, will help you narrowing your choices.
I would no expect a cheap and good Android tablet (with good exceptions) so you should keep that in mind as well.
And by the way, the performance is dictated by the hardware, like the CPU and RAM, not only by the Operating System. The Android version will run more or less fast depending of the hardware you're running.