Of course, when prioritizing projects, you will also need to look at timing and resource availability. This is taken into account in ROI calculations, but when you have three or four compelling projects, you need to look at sequencing and concurrency of projects.
I put in place an "IT Steering Committee" at my company consisting of key leaders from each department to ensure that each project was sanity checked and coordinated. Departments have their own activities and projects, and IT projects need to fit into this mix, and they need buy-in and support from the business. Conflicts of scheduling are addressed and corrected, and projects are submitted and rationalized. This has been successful for us.