For PT10 and 10.3.10 which is the last 10 version I would go with 10.8 5 Mountain Lion which was a super stable OSX/PT combo. 10.6.8 was also good but much older so get ahold of a ML 10.8.5 and you will be very happy if you do everything correct with optimizing your iMac and so on with PT10
We ran that Combo for 2 years with HD10 and HDN card and can’t remember once when we had any issues.