There are a number of Gold prospecting clubs in Denver (some are GPAA based - you don't need to be a GPAA member to attend those meetings).
I suggest that you join up with a club, or at least attend a few meetings. You'll learn a lot of things from the people in the clubs and you'll find out where you can prospect.
http://www.lornet.com/prospector/index.html - Gold prospectors of the Rockies
http://www.goldprospectors.org/Membership/Chapters/tabid/110/Default.aspx - GPAA Chapters (then select the State you are interested in)
http://goldendetectors.com/ - in Golden Colorado - they know the area well too
The GPAA is running a Gold and Treasure show Sep / 22-23 / 2012 at the Western National Complex in Denver - you might want to attend.
I use a Gold Cube - there is an exceptional amount of fine gold in Colorado, and my Gold Cube hangs on to it (but it needs to be set up right and you have to pre screen to 8 mesh). If you want to keep more gold, you may want a different sluice than you are considering. Talk to other Colorado prospectors to find out what is working for them. Every gold area is slightly different, and you will want to target your gear for where you plan to work.
I also metal detect for gold in Colorado.
NOTE: Don't assume you can dig/pan anyplace you want. Some places are highly restrictive, and others (that are active claims) would make your efforts illegal (agressively protected by the claim owners). There are many places that are open to the public (for example: A lot of Clear Creek is actively worked by prospectors).