The easiest way is to use "Save For Web" in PS or PSE.
First I flatten it (so my shadows don't go crazy) by hitting Ctrl+Shift+E, then I resize the image to 600x600 by going Image > Resize Image (in PSE I think it's Image > Resize > Image Resize or something similar).
Then go to File > Save For Web, and it will automatically save it at 72 dpi (since 300 is only needed for printing, 72 is enough for viewing on the computer). Just change the quality to 7 or 8 (in PSE, I think it's 70 or 80) and hit save.
HTH!