Rat cages depend entirely on the type of rat you are going to get. But here are some helpful bits of info to help you decide which is best for your rat.

First and foremost:

Keep in mind a rats tail. A rat may be a rather small creature, but their tail has to be compensated for as well. If you buy a cage that is too small, a rats tail will show by becoming kinked and no longer laying in a nice straight line like it should.

Aquariums:

Aquariums are OK, as long as they are big enough again. A ten gallon aquarium really is not a sufficient size for any rat. If all you have is a 10 gallon, I recommend only getting a mouse then.

If you start a rat out in an aquarium and decide to move him/her to a cage, make sure you do the transfer in an area where there wont be too much air circulation to begin with. Rats can adjust, but you must make the adjustment as painless as possible. Rats can get respiratory infections very easily if they are prone to them.

Lots of space!:

Rats like lots of places to travel, so the bigger and more elaborate, the better! Or if you only have a few small cages, find a way to connect them using zipper ties or something. Rats love having a challenging home. If gives their brains something to do. The average space needed to house one rat is 2 square feet...more rats = more square feet!

Rats are very very intelligent, so you want to make sure that whatever is keeping them in the cage cannot be opened by them. Rats have a knack for breaking out.

The bottom:

Try to avoid having a cage that has a wire bottom, plastic bottoms are the best. They are easy to remove and clean. and you dont want your rat to be walking on anything that they may get their foot caught in!

An example of a nice bottomoed cage is shown in the link below:
http://www.qualitycage.com/rat.html

Cages should have some type of coating on them, the most trusted I have seen is powder coated metal cages. They wont be able to chip away the paint and inevitably harm themselves.

Once you have the cage you think fits your rat best, make sure to get lots of cool things to put inside, like ramps, slides, and most of all...hammocks! I guarantee they will chew through the hammocks but they love them! So keep replacing them as needed. Ladders are cool too.

Here is a cool guide to some more rat cages:
http://exoticpets.about.com/cs/careofrats/tp/tpratcages.htm

A side note about rats:

Rats are social creatures who prefer to have a buddy or two. They can get lonely if you only buy one...so keep that in mind when purchasing your rat and your rat cage. The more rats, the bigger the cage.

Good luck!