It depends upon what you mean by 'modify'. There is something known as 'the Mod', which is using a CPU closed loop water-cooler for a GPU, which I have done. I have experience with the Kuhler 620, but you can use pretty much any closed-loop system on the market right now. Check out some photos I made of my own build: http://www.flickr.com/photos/64047525@N08/sets/72157628978202319/
This was using zip-ties, and you have to be careful to ensure that they're tight enough otherwise your card will overheat (mine did so, survived the experience, but be warned). You can also buy a custom-made bracket for this purpose from here: http://triptcc.com/
This will give you an idea of how popular this mod is...it works well. I play MWO, which is based on an unoptimised version of CryEngine 3, and my GPU maxed at 67 deg C in a 700D (NOT an aircooling case) in a large cupboard...pretty much worst case. You can get brackets for the Corsair series, so I guess you'd have better results with something like an H100.
Basically, this lets you use a CPU cooler on your GPU, and it works really well. I got a temp drop of about 20 deg C, and a much quieter systems, so it is worth it. However, I am now working on a custom water loop so bear that in mind...are you considering this? In which case it is better to put the money towards a custom loop from the get-go.