Here's a good link if you want to learn about serious collectors:
http://www.ccgtcc.com/
I'm not a serious collector.... I just have a few saved from places I've played, mostly $1.
I do like to use the following as card holders when I play B&M:
Bellagio $1 blue
Bike $1 blue, $2 green, $5 yellow (when not at Foxwoods)
Borgata $10 blue
Commerce $1 blue
Mirage $1 blue
and others as the mood hits