I was able to have a Revit project file open of 130 MB with other files liked into it (82 MB and 62MB files). I was using Working Sharing monitor to review my CPU and RAM usage. On opening this file with links there was still nearly 1.5 GBs of RAM available. I also had, MS Outlook, MS Word and MS Excel open at the same time. I set the file up to do a batch pdf print and batch DWG export of 40 drawings. It handled it without any problems and I had no problems multitasking while Revit was working away in the background.
In a second test, I opened up a 130 MB file. Start a render of the entire building. Then open up another session of Revit and open a 40MB file and started to do you standard day to day editing. The computer handled it without a problem.
Considering the size of the computer, this is very good performance. From this, there should not be any worry about having a file size of up to 220MB on a 4GB, 64 bit computer. This can equal a building up to about 60,000 m² and documenting it right up through Construction Administration.
It is so good to think you and get so much more out of your computer by changing to a 64Bit O.S.. Revit 64 Bit just adds to this, but there isn’t going to be any jobs on Revit 2009 that will need more than 4 GBs of RAM, especially in the current financial environment. Revit now also is a lot better at shedding RAM after closing worksets. What I would say is; make sure you still manage your worksets well. When you are giving your model to your MEP or Structural Consultants, they should only need to load in the worksets of the relevant items. Obviously the more worksets a user can have turned off the faster they will work.