I can't think of anything anybody* would want to do on a PC that could justify more than 8Gb of RAM with today's current or projected technologies. And that includes running a few virtual machines simultaneously while also working with Photoshop and browsing the web. (You little multi-tasker, you!
About the only real (normal person) 'workstation' intensive process I can think of is CGI rendering. And if you're doing a lot of that, having more CPUs (as in a small cheap render farm) is far more efficient than having a single beefed-up workstation. And that includes those that support dual chips.
Suggestion: whenever I'm in doubt or looking for solid advice on RAM, I'll web over to Crucial Memory Products
and use their Memory Advisor Tool found on the homepage. I have never been steered wrong with this tool. Also check out their forums. Some good discussions about RAM can be found there. (Note: usual caveats regarding forum recommendations apply.)
If you want to shop price before you buy, you can always take the specs from what Crucial recommends and look at comparable products from quality RAM providers like Kensington et al.
* Note: I seem to recall a few DCers (Carol or f0dder maybe?) were running with 16Gb. Maybe they and some of 'those that have' could weigh-in on this? I'm curious too since I'll need to seriously start thinking about a new build sometime this year.