1) Your best way to know if your temps are good is to run Prime95's blend test for roughly 1 hour and record the temps. If temps stay below 60C on a stock cooler, you should be good to go. Your current noted temps are within safe limits.
2) Your PSU, though most likely not best of quality, is strong enough for an 8600GT. If you start running into crashes or system errors during heavy graphic related items, you PSU might be hitting it's limit and should be looked to be replaced with at max a good quality 380W or 400W.