using two DDR-400 modules may help you getting the double data bandwidth,if your motherboard supports Dual Channel Mode. But it will never double the RAM's frequency!
Well if u r using them in single channel they will remain.....400MHz..
If u put them in dual channel..then... theortically the speed is doubled but....practically it is much much less...
Guys there's no point in burning the fingers. If it were so then why would people go for 800 MHz DDR II 1 GB RAM. Won't they buy 2 400 MHz 512 MB(each) RAMs.
lol if it were so then there would have been no existence of DDR II RAMs.