That wheelset comes with a 16t cog, though a pretty flimsy one (I have a wheelset from them). If you buy a new cog you need one like the first one, though it needs to be 3/32 for chain size, because your chainring is probably 3/32 as it comes off a road bike. No biggie. 3/32 vs 1/8 is just referring to the width of the chainring and cog. any of these will work, a bit spendy but really nice cogs:
http://www.benscycle.net/index.php?m...88_190_488_490
if not something like that, an 18 tooth cog will give you about 72 gear inches, a 19 tooth a little under 70 (68.something I think) and a 17t will prolly put you in the 75ish range.