After upgrading to 2Mbit I have found these to be my best values
MTU = 1478
RWIN = 50330
See sig' for speeds
I've found this page extremely helpful
http://www.dslreports.com/faq/tweaks Rather than spending time trying different values each time this will explain how to find your correct MTU and RWIN.
Also try reading through the "Tweek" (spelling?) guide at the top of this page.