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If you've got the first two things down cold, then you can focus on the last. If there's anything you can do to improve the user experience, that's what it takes to improve sales and traffic.
Of course, as soon as the ugliness of your website impairs the user experience, THEN it would be time to make a change.
You don't need to use Flash or graphics to make your content attractive.
flash and glitter has little to do with it
'Tis the truth. Flash has nothing to do with it. To improve productivity of your site, the single most important thing is usability (navigation and content). There are many sites out there that are actually quite ugly, but have great usability and are profitable.
Individual Flash elements on an HTML page can add some niceness too it. One site I came across had a three-column layout design (150px - fluid - 150x) and the right column had a flash element in it. It was a picture of a cherry blossom tree and it was swaying in the wind, and blossoms were falling about it. It was beautiful, didn't detract from the content (it was about Japanese influences on something-or-other), and it was just taking up space that was otherwise going to be left blank.
Some sites use Flash animations instead of title graphics. For a coding site I've been to, they had coding scrolling behind the site name in the title graphic. Very subtle (since it was dark grey code on a black background) but a nice touch.
I'm not suggesting that sites be focus only on content and usability. I'm suggesting that your priorities shouldn't rest in graphics and flash (unless that's what you're selling).
HTML + Appropriately-Placed-Flash = Goodness for All
Disclaimer: design is not about graphics and color. It's solving a problem and presenting information in the best way possible. 37Signals' site doesn't have any more or less design than Kaliber10000 (k10k). If you plan on decorating existing content with graphics and color, that would most likely end up bad.
I don't think you'll find hard-and-fast evidence because every problem requires a unique solution.