If you have less than 2 GB RAM, the earth 18 civs map that is a choice from "play now" will overwhelm your machine somewhere between riflemen and railroad. Wouldn't even recommend huge w/o the 2 gigs for avoiding late play difficulties.
Note that at Monarch and above, the difficulty in winning is increased greatly by larger maps and more civs, especially any win but cultural, space, diplomatic, and time. Worth thinking about.