I think you'd have to make it pretty damn thin for it to bend on mounting it, and there shouldn't need to be big clamping loads anyway. Bending during construction / assembly is more of a possibility with making it thin. The board warpage isn't an issue. I'm not sure of your cards final weight when finished but my 9800 pro cooler is far from light, and it's been in my tower for the best part of a year unsupported.
On removing it recently the card is a little warped now (I've fitted the cooler to the Asus 9800xt), but to be honest so what... it still works fine
put it this way unless you were told it was warped I doubt you'd notice it. The main problem with card weight is moving the case. If you move or bang the case about you could put significantly more loads on the card and agp slot, unless you have additional support.