That is a big file (~100 MB for one layer in RGB mode) and it will use a lot of memory and require constant swapping to and from the scratch partition. No way around it except to get more memory, a faster hard drive, or work at a smaller size. If you can build the poster in another program and just work on smaller components in Photoshop I would suggest that. InDesign would be my first choice, followed by Illustrator in a distant second.

And please refer to image resolution not as DPI but as PPI for Pixels Per Inch. This is the one and only proper term.

Seems to me that your PC could be the source of the problem. Can you post the specifications, CPU memory etc. While A2 size (aproximately 23.4 x 16.5 in - easier for me to work in inches) is a good size, it is roughly the same dimensions as a file straight from the D800 and without layers should be about 200 MB. While I have a pretty good system (16 GB memory with an i7  core) I can easily handle files sizes 5x this size.

Also I have an old XP machine with 4GB memory running CS3 that can, although a little slow, easily handle 400 MB files

While 300 ppi will give a good print, if you resampled to somewhere between 200-240 ppi you will still get a decent print and reduce the file size. With an A2 size print at 300 ppi coming in at 200 MB, resampling to 240 ppi would bring the file size down to ~ 130 MB and 200 ppi would bring the file size down to ~ 92 MB

MK

I think the long term solution for you would be to add memory, 4:00 GB is really the minimum, but if your system wil allow you to go beyond 4GB that would be the way to go. As I mentioned I have an XP system, pretty old, and limited to 4GB maximum.

The other question I have is how much available disk space do you have? With minimum memory you are probably seeing paging, writing memory to disk, and if you have very little free space, or the drive is heavily fragmented that could impact system performance. But the ultimate solution would be to upgrade your system, especially if you are contemplating going beyond CS3.

Also make sure there are no other applications running, shut everything down that you don't need to run photoshop. Internet Explorer, Outlook, Skype etc can gobble up a lot of memory and resources.

That's about all I can thing of, but you should consider upgrading your machine and OS if you can with as much memory as you can spring for.

Cheers

Mike

But as I mentioned, down sampling the print to a lower ppi will reduce the overall file size and may help.