I use Photoshop with a document that's about 75MB, and it takes ten minutes to open with 256MB RAM and a Celeron processor.
Celery processor is your first mistake
Actually, I'm surprised you get it to open at all. Just using Windows at all takes up at least 150MB, and Photoshop takes at least 50 more. Doubling your RAM would make such a difference that you would think you died and went to heaven.
In my job, I make Encapsulated PostScript maps all the time. One such map only took up 20MB of disk space, but it had well over 300,000 lines of EPS code! My Windows box has 1.5GB of RAM, but Illustrator10 went into a coma long before it could open this file - I let it thrash about for over an hour before giving up. Then I took the file home and tried to open it on my Mac, which has 640MB RAM. It took about 25 minutes, but Illustrator CS finally opened it. Unfortunately, switching from the zoom tool to the arrow tool was too much for it, and it told me to forget about it.
Of course this is totally off topic. This is just the story I like to tell people who wonder what's so bad about Windows. Memory management, that's what's so bad. Everything else is almost excusable. But the memory management is awful, and never going to change. Even 1.5 GB won't save you.