You need more RAM, and possibly a faster hard disk (RAID array).
Also, how large is the image?
The TIFF may be 2 Gig, but that could expand to 8 Gig of actual image data...
#1
Also check you don't have Photoshop's memory allocation set too high. The default 55% is about right for a system with up to 2GB of physical RAM.
The process of saving a file utilizes resources outside of that allocated to Photoshop.
Your scratch disk set-up is important too - especially when working with large files.
Ideally you need a good 20GB or more of free defragmented scratch space, preferably on a different physical drive to the OS. This is to eliminate read/write conflicts when Photoshop is using scratch (which it does all the time) whilst the OS is paging out to virtual memory.
Also keep the work-in-progress data somewhere other than the drive containing the scratch disk otherwise the OS will be trying to write the file to disk at the same time as Photoshop is reconstructing it from the scratch disk.
Hope this helps.
Chris.
#2