Yes, it voids the warranty. The only company that allows you to make
any modifications without voiding the warranty is EVGA.
There are two really big names in graphics card cooling - Zalman and Arctic Cooling. If you go with AC, be careful to get exactly the right version of the card. It's also possible that the right version won't fit the card anyway, if BFG have deviated from the reference design too much. Zalman's graphics card coolers are more straightforward compatibility-wise, as they are almost a "one size fits all" cooler. I couldn't say for certain that they will fit that particular card, but it would be surprising if they didn't.
AC has the advantage of exhausting heat from the card directly out of the back of your PC rather than into the case to heat up everything else, but quite a few people say that Zalman coolers are of better quality and therefore more reliable long-term.
The current Zalman graphics card model is the VF700, available as either a mix of copper and aluminum (cheapest, lightest), all copper (better cooling, but heavier and more expensive) or Fatal1ty (an extra £6 just for the name, as far as I know). If you go for this, have a quick look at how many memory chips are on your card. The VF700 comes with 8 little heatsinks for memory chips on the graphics card, which will almost certainly be enough, but you can buy extra sets of 8 if you need more.
The AC graphics card cooler for the 6800 series is the NV Silencer 5 (Rev. 3).
I can't say for certain that either will fit that particular card, but it's likely that both would.
OcUK sell both
here.