Any 7 or 8-speed chain from Shimano, SRAM or Wippermann/Connex will do. Even a 9- or 10-speed chain would work too, actually.. but they're more expensive and less durable, so why bother.
If you are changing your cassette, make sure you double check the chain length, and that it'll be long enough to handle the big ring/big cog combination without overstretching the derailleur (while not being so long for the chain to be slack in the small ring/small cog combination)
The general rule about changing the cassette with the chain is to do with wear: the chain and cassette wear together, if they're both worn over the threshold and only replace one half of it, you'll get terrible shifting and/or a really low lifespan out of whatever it was you just replaced.
If you pay attention to chain wear, you can get a cassette to last a few chain-replacement intervals. A rule of thumb is to measure 12 inches worth of chain from one of the rivets; on a new chain the rivet at the 12" point should be bang on 12 inches. If the 12-inch rivet on the chain is:
* under 1/16" past the 12" mark: the chain isn't considered worn out yet.
* 1/16" past: you should replace the chain, but you can still keep your cassette.
* between 1/16" and 1/8" past: it's pot luck if you need to change both or not.
* over 1/8": you really need to replace both.
You can also get chain-wear measurement tools to make the job a little easier.
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