I want to buy a used mac with intel onboard dual processor. Is mac pro my only choice if I seek a tower instead of a laptop?

No, you could get an iMac, they have them refurbished here
wow that is an old system you have there lol

a current gen. imac is thrice as powerful as what you have now..get it…now ! ;)

If you seek a “tower”, then yes, the Mac Pro is your only option.

However, if you merely meant “desktop”, then the iMac suggested above will meet your requirement for “Intel onboard dual processor”.

As will the Mac mini.

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Keep an eye on the refurbished units page if you’re looking for slightly used with full warranty.

I am keeping my option selection between the imac and mac pro. I love the tower aspect even though I have only added memory once. The imac doesnt make sense because I can get a Hannspree 23” monitor for $ 170. I need a newer OS, airport or wireless card to work with my ATT 2wire router (I may have to purchase a new router if airport) and update my CS to CS3. I may add Final Cut Pro (student version.) My budget is $ 600 so this upgrade is a real stretch.

sounds like with your budget, you definitely want the mac mini. WAIT it is about to be updated very soon! When this happens, you will ultimately get more bang for your buck, or you can pick up a refurb or the old version for even cheaper!

http://buyersguide.macrumors.com/#Mac_mini