Revit 2008 is a 32 bit application. And so windows allows Revit to access only 2GB of RAM. 32bit Windows can only see 4 GB. If the revit project becomes bigger than 150MB, then the project must be teasing the max memory. You can check the memory used the Revit and the total memory consumed by right clicking on your Windows task bar and selecting "Task Manager"
If you start getting fatal memory errors, you can activate the 3GB switch in your computer. More info from autodesk website. If you activate this switch, Windows will allow Revit to use upto 3GB. But your computer will be little slow. Revit uses lots of memory while printing and while upgrading the project to the latest version. More info on Windows 32 bit memory.
Some machines dont like the 3GB switch and give a lot of head ache (cannot access the internet, display issues, unpredictable Windows, etc.) In such a case, we use the USERVA switch along with the 3GB and it works great. Microsoft help on this issue here.
Windows 64 bit version can see Terra Bytes of RAM. (However this is limited by the motherboard capacity) If you install Revit 32 bit version in Windows 64 bit, Revit can use upto 4 GB. If your project needs more than 4 GB, currently there is no option.
Revit 64 bit version will be able to handle TBs of info when it is released by Autodesk! More info on memory allocation and address spaces here at microsoft website.