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As for where to buy your new computer, if you have an Apple store in your area then you can see and try all of them very easily. If not, you can still look at them online by going to http://store.apple.com
As far as choosing which one, it depends on your needs. The Mac Mini by default does not come with a monitor, meaning you can continue to use your old monitor (depending on which monitor you have, you might need an adapter) and keyboard. It's also a very small machine and is particularly quiet.
The iMac is an all-in-one machine that includes a nice LCD flat panel display, DVD drive, and CPU in one piece.
The base model iMac is 2.4 GHz with a 20" screen, while the base model Mac Mini is 1.83 GHz, so you'll notice a speed difference there too.
Let me know if you have any more questions and I'll be glad to help. Kind regards,