I had a SRAM 3x7 rear hub on my V-Rex for years, and it worked fine. Along with a triple in front, it gave me 63 gears, but more importantly it gave me a gear range of 17 to 144 inches. My only complaints were that I had three shifters on my handlebars and I couldn't have a quick-release for the rear wheel - I had to carry a 15mm wrench with me everywhere. I finally went back to a standard setup and am much happier that way. On a side note, the internal gears by themselves typically produce less range and have a slightly lower efficiency than a standard derailleur system.