Everyone has their own opinions on this. I usually post my shots about 700 pixels wide, and maybe 600-700 tall if they're portrait-shaped (rather than landscape). Quality levels are usually 80-85%, so whatever that works out to for the picture.
I occasionally post a larger one - 1024 wide - if the smaller one really takes away from the quality of the photo.
On my site, I upload everything at a max of 1600 wide or 1200 tall, and it automatically resizes to fit into 1280x1024, 1024x768, and 700x600. That usually gives me a good range to work with. I suppose maybe the smaller size should be 800x600 to match potential desktop resolutions and give a little more detail to the most commonly-used sizes, but if I do that, my Gallery software will put all those in a new location and I'd have to update all my posts that link to them... no fun!
Ideally, we could post everything at, say, 1280x1024 and give plenty of detail, but that breaks the layout too easily for vBulletin. I have a mod for my forum that resizes all photos (using Javascript) to fit inside the layout, no matter how large they are. If they're too big, you see a shrunk version, and clicking on it will show the full-size in a new window. I'm not sure if such a mod is available for vBulletin (I use phpBB).
It's kind of a shame, photos that look great at a higher resolution usually lose a good amount of appeal when shrunk down to a forum-friendly size. But there are no easy answers.