I voltmodded a 7900 GT and a 7950 GT, with no problems.
The 7900 GT however, died when I put in a friends PC with a weaker power supply (at the time, he had a more powerful CPU, and I wanted to see what 3dmark score it would get)
Moral: Always, always, always get a good quality power supply with adequate power for your components. Especially if you plan to voltmod.
A friend of mine tells me, however, that the 7900 GT might actually be fixable.
I do actually get a display from the 7900 GT for a few seconds before it cuts out. Which would suggest a capacitor or a resistor might be the problem. They can be replaced with a soldering iron.
Also, when I first got my watercooling, I also got a waterblock for my 7950 GT.
When I turned on the computer, I just got a black screen.
I found out why when I took off the waterblock. It wasn't contacting the GPU properly (even though I was sure I put it on right)
There was a bubble next to the GPU. It melted.
That's when I got my 8800 GTS 320MB.
Basically, voltmodding the cards for me went fine. the 7900 GT I got all the way to 750MHZ, and the 7950 GT I got to 730MHZ (though at a slightly lower voltage).
They did die, but not because of the voltmods (or in the case of the 7900 GT, not directly. But it put too much strain on the power supply. But hopefully, I can fix it.)