Isn't 600 watt a little underpowered? Higher powered ones don't cost that much more or take up much more space. Make sure you get one with a turn-table. You don't really need one with all kinds of "settings" though.
Is there a specific reason 600 watt? Does it have to do with power availability or maybe the room requirements?
You probably wont get much more that hot water or popcorn with a 600 watt. I just recently purchased a new microwave and was told to get at least 1200 watt for my usage. I use to cook items as well as heating.
As far as where to go, yeah Walmart is generally the cheapest, unless you have an ABC Warehouse around, they will work with you on the price. The dropped mine from $117 to $77 and threw in a NEW coffee pot when I bought the fridge too!