CLUSTER will eat maintenance_work_mem during index rebuilds --- more or less. You shouldn't expect these numbers to be dead on, particularly not in older releases. It looks like your 2Gb spec has turned into 3.6Gb actually eaten, which is a bit sloppier accounting than I'd like but not overly surprising. (FWIW, I think CVS HEAD is better than 8.0 at measuring sort workspace.)
regards, tom lane
Thanks for the clarification. It doesn't sound like it's going to jump to 8Gb and hog most of the RAM.