Hi there,
In addition to the main larger web based travel sites, I'll suggest a couple more that I've used recently to get really good priced flights. As per usual, when it comes to hunting for the best price you can get, for me I find that you must keep checking individual sites time and time again over a period of about 2 days and keep comparing the prices against one another. On my recent trip to Vegas in early Novemeber 09, I did this and as I had multiple flights LHR>ATL>LV and then LV>chicago>cincinnati then cincinnati>ATL>LHR - I found the best flight price of around $950USD for the lot via this site
http://www.airlineconsolidator.com/On my most recent trip to Vegas again, 2 weeks ago, I had a similar multiple stops requirement and I paid around £740 GBP via Lastminute as the other site was coming up with allsorts of convoluted routes and added silly 8 and 12 hour stopovers.
~Another site is -http://www.cheapoair.com/
These both are US based websites so keep checking them and comparing to say the Kayaks, Expedias, Lastminutes etc of this world. Between them all, you should find a good priced fare.
Hope this helps!
Cheers, DtM! West London & Slough UK!