You won't "fry" your card, just like slightly increasing the voltage on your processor won't "fry" that either--in the sense that it won't cause a big puff of smoke and immediate catastrophic damage. Over the LONG run, more voltage means more power and more heat, and so it's worse (all things being equal) to have higher voltage. But if you want to be more conservative, change only the clock speed and not the voltage. Higher voltages tend to allow greater stability, though, so it means you'll have a lower OC.
Also, adjusting the voltage will slightly increase power consumption by itself, but much of that 20W figure comes from higher clock speeds, too. Since the card will have a variable clock speed, you won't always be using 20W more, but you will be using slightly more than you'd otherwise be using with no voltage adjustment. It's like having another cylinder in a car's engine--it burns more fuel at all times, but if you're only at 1500 rpm, it's not that much more. I'm not 100% sure how the voltage on the 7000 series works and how it changes with clock speed, but it tends to be less variable than the voltage of, e.g., a Sandy Bridge CPU.
Sorry, forgot my bottom line point: if the voltage increase is small, the only difference between an OC card and a non-OC card is the heat. The 7000 series runs very cool. Even at a huge OC, you won't break 80C. So you won't fry your card.
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