It's really depends on what you are looking to show.
With 300DPI, you can create a great looking print. The file size will be huge....
If you are trying to post something for your for sale ad, 75DPI is more than sufficient.
Most of my gallery postings were done at 150DPI or less.
You also have to deal with compression ratio for creating jpegs and it has as much impact to the file size as the DPI. Frequently, they are labeled something like "best" or "minimum compression" for good quality (read big file size) compression and "small file size" or "maximum compression" something for most compression.
If you compress too far, file size will be smaller but you'd start to see loss of detail in not-so-textured areas like sky and deep shadow. I tend to pick somewhere in the middle - depending on the size of original and scan DPI.
Bottom line, though.... for online galleries, unless you are going to be judged or someone is going to view it critically, it really doesn't matter. It's hard to tell them apart.