There are two questions that need to be asked and answered by you first:
1. What do you want your PC to do. i.e. gaming, video editing, etc.?
2. What is your budget?
Once you answer those two questions the next question is, why do you want to upgrade your motherboard? My own experience working with Compaq/HP/Dell -- other people's experiences will vary -- is that their cases are fairly prepriatory, especially older models.
In other words, they are often smaller than a standard mid-tower case and to upgrade can be a very unpleasant experience.
My suggestion: Discipline yourself to save money and build your own. This way you can do solid research and choose the parts you want, plus it makes upgrading parts a little easier for the future.