My brother's name is Richie. He's 12 years old and since Richard is such an old fashioned name, I've never met anyone named Richie. I know that Richard is kind of popular for older men, though.
This is now my nickname. My name's Michaela but I got to Richie like this Michaela Kyla Kylie Minogue Singer Famous Rich Richie! It's fun! And Richie sounds fine for a nickname for a girl :) just not a name!
I know someone named Debra Richardson, but she's known almost exclusively as Richie. So I think it's really cute for a girl, as long as it's not her given name.