The lifetime warranty is mostly a marketing gimmick imo. Most cards have 3 year warranties and that more than covers the average useful lifespan of a video card. For example 3 years ago a $150-180 card would have been something along the lines of an 8800GTS 640mb. If you still had one now it would be a mediocre card for low resolutions and you likely would have already replaced it and if not then its warranty running out and it breaking would just be a looked at as a good excuse to finally get something better.
As for your other question not just all the XFX HD5770s have the same processor but all of the HD5770s in general are the same processor. Physically the cards differ only based on cooling and perhaps a factory OC which, as you point out, isn't worth paying extra for. The cooling is actually more important as to how fast you can get the card to run which is why I listed the cards with the best cooling. That Hawk card in particular seems very good for OCing with a lot of people getting it up to 1000-1050mhz.
Another option would be spending bit more for a better card. The GTX 460 in particular would be something to look at and a nice step up from the HD5770. It can also OC a very large amount, 30-35% even on the reference cooler.