Check out Chris Botti. He's a trumpet player....I've got 2 of his albums and listen to them at work when I need to kill some stress. It goes good with Maker's Mark or Scotch too...
First Album I got was called "Midnight Without You"....named for the only song on the album with lyrics. Alone in the City is a really good track, as well as the Steps of Positano and others. It prompted me to get another album, "When I Fall in Love" which had more well-known songs. My only gripe is that he has a version of Ordinary Love (old Sade song) that has piercing high muted high notes for the chorus on the trumpet. I basically reburned the CD without that song and it improved the album greatly. I like iTunes because you can just uncheck the sucky tracks.
I was going to suggest Diana Krall (noticed someone else did). I like the music she sings, but I'm not a big fan of her voice. While she's definitely decent looking and has a good voice, her accent and pronunciation of certain things makes it so I can only handle so many tracks. Whereas with the instrumental stuff, I can listen to it for much longer.
The REAL Jazz singer is Nat King Cole. He has the voice, the proper voice accent and pronunciation in his words to make you believe he means what he's singing. Aside from him, you know Ella Fitzgerald is the other serious jazz singer....and she knows how to tear it up.
Here's a cover of the Nat King Cole style....
http://www.youtube.com/watch?v=uikuFx-TCEI
And Ella.....
http://www.youtube.com/watch?v=1j6avX7ebkM
Now....for something else, I HIGHLY recommend checking into Stephane Grapelli. He is a jazz violinist. He got me hooked on that style and it's more upbeat than the slower mellow stuff. He's just very gifted. Be sure to give him more than one listen. He played for many years and some tracks available are definitely better than others....but many will be faster tempo than what you're looking for.
Here's an example of his stuff...
http://www.youtube.com/watch?v...WxIIz8&feature=related
http://www.youtube.com/watch?v...25JG4I&feature=related