i agree with the notion that a 600EX would get you the most functionality for your dollar with the parameters you stated. replacing the 24-105 or the 70-200 would be nice (both replacements suggested would show improvements but it would be nominal) but the cost to do so would be far greater.
if you simply had to get an additional lens i think the 35mm F1.4L would offer the most benefit to your existing kit. that lens is simply an All-Star in canons line up. can be tricky to use with moving subjects wide open, but well worth it if you have the patience to learn to use it.
back to a speedlight....
+100 to the idea of properly using bounced light for low light situations. i would even suggest that you get a radio trigger system to plug into the speedlight so that you can use the flash off the camera. i attach AB CSRBs to my flashes and trigger them remotely with the CSTs. this would allow you to place the flash anywhere in the room (bounce off the ceiling) and effectively raise the light level of the room in a more natural way.
600EX + CSRB/CST combo would run you @ 725.00 and increase the functionality of ALL your existing lenses (and any future lenses you purchased). keep in mind you would also need to buy a hot shoe adapter (15.00) to attach to the bottom of the speed light so you can plug in the CSRB.
Thanks for the speed light advice. I will plan on looking at that setup. Would mind pm'ing me the parts your suggesting. I think I am going to evaluate the 35mm after Christmas. I still have the 50 and will work with flash. My only problem is the temptation to spend the extra $850 for the 70-200 2.8 is ii and return my f4is. I don't want the weight but I can't help but wonder if I will upgrade later and if that will make better use of my af system. Any thoughts this is regardless of the speed light I am pretty sure I am getting.