I read through the Quick Start Version 2.0 paperwork and couldn't find the guarantee to which you refer. Nor could I find such a guaranee on the outside of the box. Where did you see this?
I only have personal experience to report, but I think you are very lucky indeed to be able to crank the memory to 750MHz. Altogether I've had four 4600 cards pass through my hands, and none has reached 750MHz with any degree of stability! Or maybe I was unlucky.
In point of fact, my own Ultra/750XP had to be replaced because the card would not run anything more than 'Safe Mode Settings' on the performance panel (300/648). I did not mention this because Gainward replaced the card without question and their service support was absolutely first class. The replacement card works fine but even on the KR7A it is distinctly unhappy at anything much above 680MHz.
The primary points I wanted to get across were simply:
1) The Gainward card is feature-rich and has superb picture quality
2) Avoid pairing this or any 4600 card with a GA-7VRXP because this combination has many reported issues (and mine plain doesnt work)
3) All 4600 cards are likely to be rendered obsolete by the new ATI chip, so the primary advantage of the Gainward has to be its features
If I had to buy a new graphic card today it would be the GF4 4200 from Gainward (for the features and price), and tomorrow it would be the ATI 9700