With an 18.5" LCD I am assuming that you will be running at 1280x1024?
If so, the 4850 will be approx. 37% faster and the GTS 250 will be approx. 40% faster on average than a 5670.
Your best 3rd and 4th choices would respectively be the 4830 and 9800GT which are 15-19% faster on average.