Here's my "workflow" for resizing for posting with a 100k filesize limit (i.e. here):
1. Image/Resize. Type in 800 in the first box (sorry, don't have PS with me to verify). The next box should change to 533.
1a. Any post-processing (I'll usually give the pic a little USM, maybe 90/0.3/3. Any other things like levels, etc.
2. File/Save As. Always rename. Don't overwrite your original file (especially if it is your only copy). Give it a name, make sure JPG is the target type. Another box pops up to ask what compression level you want. If preview is turned on, selecting a compression level will give an estimated file size at the bottom. Adjust until the size is 95k or smaller (it usually underestimates a bit).
3. Click OK and you now have a new, renamed file, suitable for posting here with a filesize under the 100k limit.
Enjoy!