+1 Shooting in RAW
+1 raising your ISO, you're going to probably still run into problems with the 100-400
Get a tripod, get a decent tripod,...saying that you'll have to make do with the two lenses you have, as far as focal length goes, is a big overstatement, you have pretty much the whole range covered there other than super wide and super telephoto.
A decent tripod is one area where people try to skimp for as long as possible, in a lot of cases, next to glass, a tripod should be 2nd on your gear list,...that's right even above a decent body. I shoot with a 350d but I have a pretty serious tripod under it pretty much any time I put my 75-300 on. This frees me up from having to shoot at ISO 1600 or having shots that will give you motion sickness. Don't skimp on the pod,...you'll thank me, for years to come.