Help me pick out music for a party

"Big Balls" by AC/DC

Pay more attention to the mood you want to create than worrying about a particular genre. If the main activity is expected to be conversation, pick something upbeat, but not too rocking or distracting. I find Dave Mattews perfect for this. Mellow enough to be able to talk over, intersting enough to listen to if someone gets bored with the conversation, jazzy enough to sound sophisticated, and generally upbeat and happy.
Also, NEVER LET THE MUSIC STOP!! I'v seen several times when the music stops and people start leaving before the host can put another disc in. It struck me as strange. Even if they don't leave, they will feel the change in mood, and feellet down (if you chose well), or relieved (if you chose poorly or played too loudly). You really want to keep the volume down very low.

Try This,

Koop (name of artists) "Koop Islands".

It's found in the electronica section of HMV etc, but it isn't dance or electronic music.

On the cover, it says (paraphrasing)... "music reminiscent of 1930's swing/big band era that can be heard in the dining hall of a liner while on a cruise in the Caribbean"... or something to that effect.

The first track: Koop Island Blues is awesome. Most of the tracks is sultry female vocals with rhythmic brush ride cymbals in the background.. and an airy summer Caribbean flavor.... upbeat and interesting.

If you're open to something more exotic, Cafe Hammam from EMI is a collection of relaxed middle eastern music, mostly acoustic; piano, sax, violin. Not bombastic like much middle eastern pop music (which I love, too), but suave and sophisticated.
I also like the sound of Elf73's suggestion, although I've never heard it. His description makes it sound perfect

You say that most people are in your age group and probably share your tastes,so unless you are hiring a DJ, play the music you own and know. I would probably play lighter, happier music from the time periods you cited. Use the "Big Chill" as a guide. Definitely throw in some 60's Motown.