OK.
N adapters are technically the "best" adapters out their, considering the strength of the N signal..So I'd buy a N Router and a N adapter. Of course, B/G is available too, but it has speed limits, but I forgot what they were. N has a higher speed cap..Meaning, faster Internet, if you support it.
I recently had to buy a N gaming router due to the speed increase of my ISP. B/G were no longer supported at the speed I have.
Anyways, for the past 5 years, I've bought 2 Linksys USB adapters. One lasted 3 years, one lasted 2, before I broke it. They aren't exactly cheap, but very reliable.
http://www.linksysbycisco.com/EU/en/products/WUSB600N
That's the one I have now, but your store may not. Pretty much, just go for anything N, if you plan to buy an N router. Go for B/G if you plan to buy a B/G router. The reason for this is because although N routers are backwards compatible with B/G devices, you'll get a better experience from using the same signal.