I have 2 Quads, and both run 24/7 @ 3.6GHz. I've managed to boot one at 4.2, but that was a suicide run. Now for your OC, 3.6GHz with a Quad under 50C on air is VERY respectable. You should be plenty happy with that. I would not try going higher 24/7, leaving yourself some headroom.
As for stability, run Prime 95 25.3 (
XtremeSystems Forums - View Single Post - New Prime95 v25.3 Full Support for Dual/Quad Core + HardwareMonitor v4.3.1.2) to have all 4 cores at 100% - Orthos is using only 2 cores. Now you may see some higher temps. Max. is 1.45V and 60C for 24/7 use, under load. Leave the Ram out of the equation by running 1:1 ratio and 2.0V; oc it later, once your CPU is stable.
Use Coretemp to keep an eye on temps.