Be very careful replacing UPS batteries. The brand DOES matter! I tried using a Universal Battery battery (from Batteries.com) in a Back-UPS Pro and the thing started making some not so good noises (pop, gurgle, pop) when it was charged. That battery is currently functioning as a doorstop.
The only 2 brands I have ever seen APC use are CSB and Panasonic. Though I'm not 100% sure but I think the Panasonic's have a much longer life than the CSB's. If you want to go the safe route, get an APC certified battery. You can usually get a good deal on them when Dell has one of it's 20% off + free shipping days.