Have you defragged the drive lately? Possible this might help.
Am told also to turn Virtual Memory on (Macintosh). Apparently Photoshop
uses all the RAM you have (I don't know why 1 GB isn't enough!!), then
begins to use the hard drive space as memory.
If HD is too fragmented, or is just plain full (of all the other stuff on
your computer), that's a problem. But you probably already know this.
(Currently avoiding memory problems by having only small files - like
that's a real option for anyone! *sigh*)