Many dancers perform all styles of belly dance in a standard bedlah or two piece belly dance costume, but it's nice to see an increasing number of dancers opting for a proper beledi dress for saiidi and beledy choreographies.
The traditional garb is a loose cotton shift, but modern versions are figure-hugging, made of stretch material and sometimes low-cut enough that you can show off a glittery bra underneath. There are subtle differences between a baladi dress and a saiidi dress - the sleeves on a baladi dress are open from the elbow, for instance - but for performance purposes, they are pretty much interchangable.
Not clear what Baladi is? There's a great article on Belly Dance Stuff here.
Rania Renee has a wonderful DVD on Progressive Baladi