Apple Icon > About This Mac
Mac OS X Version 10.7.5 Processor 2.4 GHz Intel Core 2 Duo Memory 2 GB 1067 MHz DDR3
Apple Discussions say that macs have supported 16gb ram since 2009, but that you only need more than 8gb unless you're running virtual systems for doing huge photoshop stuff.
Carbon Computing says they'll configure to order including custom ram, but don't list individual part prices. I sent an email to their sales team.
wikiHow says that a 2011 mac mini maxes out at two 4gb sticks, or one 8gb stick, and that it must be: non-parity, unbuffered, 1333 megahertz (MHz), PC3-10600 DDR3, 204-pin module. Don't buy RAM with buffers, registers, parity, error correcting code (ECC), phase locked loops (PLLs), or extended data output (EDO).
Mac Rumors had a bunch of dead links for ram kits for sale.
Cabon Computing replied "The maximum amount of RAM supported by your system is 8GB. We sell compatible kits for $79.95 (2x4GB) and we have many of these in stock.", so I bought that.