The best aftermarket cooler out there is Prolimatech MK-26 Multi-VGA Cooler
, but for sure is an overkill for a GTX660ti, that said you probably can cool it with fans spinning so low that will be inaudible under load, but its a big cooler that probably wont fit the R300, i couldnt find the height the case can fit.
Artic cooling has a a lot of coolers, Artic Accelero Extreme III is their top end aftermarket cooler, but also an overkill for a GTX660ti, they also offer other models, like Accelero Twin Turbo II
, but i dont see the GTX660ti as supported, as many others, you could write to them.
Another alternative, if you could sell your GTX660ti, is to get another the GPU, a MSI GTX760 N760
should be more powerful and by the reviews its gotten, seems a very quiet gpu.