Preview does what you want, as does Graphic Convertor.
Memory really has nothing to do with the amount of space taken up by your photo's. The important thing is the amount of HD space that is required. 1500 photos in iPhoto would not really make a dent in the HD capacity of your 24" iMac, regardless as to the amount of RAM you had installed.
What was the name of the program that took all your photo's and compressed them into one file?
.... that does not create scratch files every time a folder is opened. If I want to open a folder with say 1500 photos with iPhoto, or one very similar to it, all the files are read into memory.
I got religion about a month ago and bought an iMac with a 24" screen and 2 GB's of memory. I thought this would be more than sufficient for my needs. I had a program running in XP that would take all the graphics in a folder and compress them into one file. This meant that 1500 jpeg files would be placed in a highly compressed format that would would be in the range 12 MB's. The photos would then load from this file. This was all done on a low end Dell PC with only 768 MB's of memory.
I only need a basic graphic program that will allow me to change brightness and contrast and rotate files; nothing fancy, but that does not create scratch files.