By Narae Kim SEOUL (Reuters) – As South Korean rapper Psy readies what he hopes will be his next hit song, aiming for a repeat of the viral "Gangnam Style", retailers are keeping a close eye on what he wears, hoping another blockbuster will send sales surging. From the tortoiseshell sunglasses to the two-toned Oxfords the chubby singer sported for his YouTube megahit, a wide range of brands – and Seoul's Gangnam district itself – have reaped rich rewards from the Psy effect. …