There are things you can do to get a lot more shots out of the battery, too.
1. Set the "sleep mode" to come on after as short a time as possible. The 50D wakes up from sleep mode almost instantaneously, so it's not a problem having it set to go to sleep quickly.
2. Set it so it doesn't automatically replay every image. Only call up images with the review button. I also assign my "Set" but to "review", so that I can call up an image with either thumb while shooting.
3. Avoid using the built in flash. It really hogs power. And it's pretty lousy anyway.
Modern batteries, and third party especially, are often higher powered than the original BP511As that were sold with the camera. But your old batteries might perform better after a few drain/recharge cycles, if they have been sitting unused for a while.
I use battery grips on my 50Ds, so two batteries. And I always carried a couple spare sets. I am pretty sure could shoot around 1500 to 1600 images with each pair of batteries, freshly charged. That was with the cameras set up and used as described above (I.e., I never use a built-in flash, etc.), but using both AI Servo focus and IS a lot. Those aren't heavy power users, but they use some.