PSU/aftermarket cooling to maximize component lifespan

Hello all.

I'm hoping someone could answer a question about Power supply lifespan. If you run a well made power supply rated at a much higher wattage than your actual power needs does it significantly extend the lifespan of the PSU before it's performance starts to deteriorate. For instance an Antec TruePower Quattro TPQ-1000 1000W PSU in a system running a single 4850, an intel e8500, an hd, 4g ddr2, and 2 optical drives(just an example no need for specifics for that particular Configuration).
My interest in this would be in terms of upgrading to crossfire/sli later adding more ram and possibly eventually overclocking.

Also, what effect if any would upgrading with aftermarket or water cooling have on the lifespan of components that aren't overclocked.
I'm thinking about ordering a new system in or over the next couple months with an eye towards long term stability and upgradeability.

Thanks in advance for your input.