Battery grips are the bomb. My XTi came with one when I bought it used I picked up 2 more batteries and a SD-CF card adapter and now I can shoot for days on end without worrying about lack of battery power OR lack of storage. My current lenses are the kit 18-55 non IS which I barely use, 50mm 1.8 which is my standard prime walk around lens, a 28-105 USM II that is my walk around zoom lens and then a 70-210 F4 that I use for applications where I need the extra reach (air shows, wildlife photography, it's an awesome macro lens too).
My advice would be to ditch the kit lens and instead hunt for a used 28-105 USM II as a walk around zoom. Second lens I recommend to anyone that's shooting Canon is a 50mm 1.8 or a 1.4 if you've got the $400 or so to burn either way is amazing the 1.4 gives you a couple stops more power but other than that it's identical to the 1.8. I know Mike Deep will massacre me for this but I LOVE my 70-210 F4 granted not the fastest autofocus on the planet and can be a little soft wide open but if you stop it down even a 1/3 of a step you get some great narrow depth of field shots. Best useage outdoors for it is around F8 where's it's exceptional and not to mention you can find them on ebay for $125 or so all day.