I had a similar experience. open RP last June with a positive margin --- Swim is right (as usual). You may have to do nothing, but do the ultral sensitive PSA as your doc directs. Mine bounced around between .04 and .09 for a few months, then jumped up to .24. My best research indicated .2 as the point where you want to act.
When I consulted with my onc radiologist, he was more concered with my age (46), pre-surg PSA (39) and tumor bulk (lots!) than the PSA recurrence. I just now complted 36 sessions of daily IMRT radiation, and am on 6 months hormone thereapy.
Happy to report MINIMAL side effects from both --- heard a lot of "horror stories", but no real problems -- just an inconvenience. There are also a hand full of proton therepy facilities, but I didn't see a huge advantage for salvage/adjuvant radiation on proton vs IMRT.
The only additional advice I'd give is is don't be afraid to consult with a radiation oncologist just to get their take on your situation, and to see at what point they'd treat you...GOOD LUCK