The higher band width would help with overclocking the e6400. If you decide to go e6600, i would just stay with the ram you have now since it will be easier to overclock.
But i personally would just keep the e6400 and get another stick of ddr2 667 or sell your current stick and get some value g.skill ddr2 800 ram (or some xms2 if you get it while it still has a rebate). You probably won't notice a BIG difference between the e6400 and the e6600 if you were to overclock them to the same speed. If decreasing boot time is your goal, i would do what i suggested with selling your current ddr2 667 ram and getting 2gb of ddr2 800 so you can overclock your e6400 more.
Like my boot time is around 30sec (i can't seem to get the bios to stop checking the dvd writers every boot up for some reason even though i took them off the boot list but w/e), and i have my e6600 set to 425x8 which your e6400 can do w/ a good hsf and ddr2 800 ram (my HZ's are overkill for me cause they can get to ddr2 1067(according to anandtech)).
Hope that helps, sry i write a lot i can't seem to help it anymore after writing so many college essays