Tristan is being used alot as a boys name now. In fact, I know 3 male Tristans, lol. As for Avery, that's beautiful and was actually on my list but dh didn't like it for some reason. He gets veto power but I get final choice, lol.
The Social Security Administration actually has a site that can give you the popularity of baby names. The latest data is from 2007 but it helped me to trim my list by quite alot. Here's the link.
http://www.ssa.gov/OACT/babynames/
I looked up Avery for you and it was ranked 48 but if you spell it Averie then it jumps clear up to 823.
Tristan is in the top 100 also.
I love that site!