how much of a difference will there be between pushing 500watts rms into a 500 rms sub or say 800 watts into a 500 rms sub? also how much more stress does this put on the sub?
the RMS power handling of a speaker is a thermal rating that tells you how much heat, measured in watts, the voice coils can handle without being damaged. This isn't an indication of the power required to mechanically reach peak output. That figure, though rarely stated, is usually less than the RMS value of the speaker.. usually around 80% or less of the RMS power handling rating.
Overpowering a speaker is asking for coil damage which, when the coil fries, can cause a short that fries the amplifier too.
now a "500 watt" amp (RMS) only puts out that power continuously at full output, or full volume. every time you halve the audible volume from full blast, you're using a tenth of the power of the previous volume. (power the audible volume is a log scale, not linear.) so a 500 watt amp may never really put that much power to the sub if you don't crank the stereo to full blast. something to keep in mind.
I wouldn't go past that 600 watts on your sub in short. 600wRMS from the amp is still in a pretty safe zone. particularly if your listening habits are a bit more reserved than say, Pete Townsend's when he used to stick his head in the Marshall stacks at Who concerts.