get a basic card and overclock it if you are overclocking AT ALL. Of course, you need to take in consideration of temperatures (look at the gpu heatsink, is it a better non-reference one?)

Does overclocking a video card that is already factory overclocked put more strain on the video card as opposed to a basic version of the same card?
A: If both are at the same clock, no. Both experience the same amount of "strain."

I recommend EVGA Precision or MSI Afterburner (which I use)

Your overclocking profiles can be set to be applied during startup.

There have been problems with gtx 580 cards burning up from large voltage adjustments. Keep it safe!

asus is too release a new software called GPU tweak that will make overclocking very easy soon enough

You are paying a premium for the company to OC the card so when you buy it, it is tested to work stable at those frequencies right out of the box. Your best bet would be to save the money, and get the stock version and OC it yourself. The pre-overclocked cards aren't even OC'ed very high, the change are very minor and in most cases the card can be pushed even further.

Let me throw in my 2 cents. It is senseless to pay a company on average 40-50 dollars extra to overclock a card for you, when you are just going to overclock it anyway. I don't see the point in wasting the money if they both go the same distance.
But that being said, overclocking is not just about finding how far you can go in terms of MHz, it's about finding that sweet balance between memory and core clocks that seems to unleash a whole new term of speed.

Example: My old 9600 GSO 768MB. I used to just max everything out and I would gain 15 frames on average. Then I started taking the core back down and edged the memory speed forward another 5MHz. That seemed to unlock some sort of magic number where I, all the sudden, had 23 frames more than stock clocks would give me, which is more than when the whole card was maxed out also.

Buy a standard card and find the sweet spot.