Phono Newbie

After saving my pennies for quite some time and with much appreciated help from others here, I recently purchased the following: AR 300.2 amp, AR LS17 pre-amp, and Magnepan 3.6 speakers. I have a small collection (about 50) of vinyl from the mid 60's to mid 70's that I would like to listen to. I also have an old Onkyo turntable (circa 1980). I need a phono pre-amp, but am at a loss when I read terminolgy that includes things like 47k cart load, etc. Since my vinyl collection is small, I want to keep my phono/pre investment modest. Where do I start? Is my Onkyo still usable? What should I look for in a phono-pre?