How can I benchmark my new RAM?
I've always used Memtest86 on a bootable disk to do memory testing. Not sure what you mean by "burn in" though. Other than testing for stability, bad block and general good-functioning of your RAM there isn't anything else you need to do. Using the RAM doesn't actually make it perform any better or worse than when it was initially installed in your machine.

Burn Memtest86 to a CD. Boot from the CD. Let it cycle through all your blocks and it will produce a report.

There's an OS X build of Memtest that's available through this OS X Daily article but I've never used it. Can't speak to how well it works.

There's also memtester and it's available as a Homebrew package:

&gt; brew info memtester
memtester 4.2.2
http://pyropus.ca/software/memtester/
Not installed
https://github.com/mxcl/homebrew/commits/master/Library/Formula/memtester.rb


To install it:

&gt; brew install memtester


Again, haven't used it so I can't speak about how well it works, but it's a userspace program so it means you don't have to reboot your machine to get it to work. I'd probably start with that and if it doesn't give you a suitable report, move on to Memtest86 on a CD.