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.
5D3, 5D2, 5DC, s15mm Fish, 24mm TSE, 35mm F1.4L, 50mm F1.2L, 85mm F1.8, 100mm F2.8L, 24-70mm F2.8L, 70-200mm F2.8L, 580EX, 580EX2, 600EXRT