[gojomo]

Jakob Nielsen's Useit.com has lots of good, test-discovered guidelines in general. Though not specifically aimed at 'download'/'purchase now' buttons, all of the "Top 10 Mistakes" articles are worth reviewing. You could start at:

http://www.useit.com/alertbox/9605.html

Jakob's "Law of Web User Experience", which is "users spend most of their time on other websites", is also a good guide. Study and mimic the best practices of other more popular sites that use the same sort of 'download'/'purchase'/'add to cart' button. They've done the testing and trained the users; novelty usually confuses.