How full's you're hard drive?
Previous versions of Photoshop make a lot of use of temporary fake ram in the form of your hard drive. (Pardon me, it's late and I've forgotten the correct terms....page file maybe?)
If you're lacking space on the drive (often specified in it's options as a scratch disk) Photoshop can complain.
If you've got a spare hard drive which is not part of the boot system and it's got a fair few gigs free, you could make that your primary scratch disk.
Or, ideally, if you create a partition which you use purely for Pshop's scratch then it has a large chunk of contiguous fake memory to play with.
[this may of course not be the problem at all, but it's a thought]