Haven't got it installed at the moment (hdd failure, yummy!)
Mozilla's currently taking 38 of 512mb total, and thats with the IRC client open, and 3 pages/tabs.
I should imagine Opera will be slightly less.
One thing to bear in mind with resource uses, is that IE is an integral part of your operating system, while the data contained in the webpages is the same, the initial footprint will be smaller.
Opera and Mozilla (and all their friends) have to load their own rendering engines etc into memory before any external data is even retreived.