the amount of heat it gives off is the same for every step that you raise the vcore... i wouldn't go above 1.45V on stock though.
use speedfan to check temps.
the amount of frequency you get per volt does depend on the cpu though.
it wasn't money wasted, the FX series guarantees high quality cpu... higher than most. i can only o/c my proc to 2.6ghz, and thats at 1.5V. the FX-60 has more cache than my cpu and guarantees 2.6ghz at 1.35V i'd say 2.8ghz should be where you'll end up... if not 3ghz. its been done with X2 3800+'s before... but its much less likely with an X2 that the FX-60
it all depends on how much the CPU speed is worth to you (some people are real enthusiasts and have the money for it) the FX series has unlocked upward multipliers which is what makes it worth it to some people. myself... i think dual core is definitely worth the upgrade, but the extra $700 for the FX-60 is definitely not worth dropping. my cpu didn't even cost HALF the difference lol