Check it. Hit CTRL-ALT-Delete and pull up the task manager. It has a tab called Performance, with a memory chart. Let it run for a bit and see how your mem usage goes. Mine tells me that 1 gig is plenty.
EDIT: With TradeStation 7.1 (with a lot of data), IE, Outlook and Task Manager up, I've got just under 600k physical memory available. TWS might take some, but not much. My trading app will take some, but not much again. 1 Gig is more than I need. I dunno what apps you are running nor how much you are pushing them, but in general 1 gig will be more than enough memory for most traders.