On some earlier Epox boards the 5V supply from Enermax PSUs was dropping to 4.7v or lower,there was a mod for the Enermax PSUs you could do to bump it up to 5v , the low 5v drop happened with some Epox boards,I`m using an Enermax 431w at the moment on my Epox 8KHA+ and get a constant 4.98v( well within safe spec) with no problems.
If the 5v supply drops too low then that will effect stability,however if you overclock that will also put some extra strain on the 5v rail so having a good PSU is important,I must say that the latest Enermax`s seem to be better then the earlier models.