Open Windows Task Manager. Go to the Performance tab. The number to watch
is in the Physical Memory section, "Available".
Start Photoshop and start working. That Available number will decrease and,
after a while, will often stabilize out.
Is it below 15000 (15MB)? Your Photoshop memory percentage is set too high.
Lower it and try again.
Is it above 50000 (50MB)? If you really have been doing things you normally
do, including running a filter and you still have more than 50MB free,
you're probably leaving a little performance on the table (but not as much
as you think!). Consider increasing Photoshop's memory percentage slightly.
If you *really* want to get technical about it, bring up Performance Monitor and set it up to track a few things (free memory, disk activity, memory paging rates).
What you are trying to avoid is having the OS page out Photoshop's memory.
We don't lock down Photoshop's memory because that causes many, many more
problems than it solves. But when Photoshop has allocated a lot of memory,
some of it looks to the OS as "not busy" and will get paged out if RAM gets
low. If Photoshop's scratch and the OS paging file are on the same physical
disk, this is doubly bad.
Windows XP Pagefile Settings:
You may want to defrag your boot drive and make sure the minimum XP paging file size is at least 3GB. I usually like setting the minimum paging file size == max size == 2x RAM.
You'll find the settings in the System control panel, Advanced tab, Performance button, Advanced tab, Virtual Memory button.
If all you want is the fastest performance on your machine, this is all you
need to know.
For further information on how to optimise Adobe Photoshop, see: