Salesforce Nonprofit Cloud is an interesting option, although some organisations may find it more than they need. I have also come across Bloomerang and Raiser’s Edge, and each has its own strengths.
For me, good activity tracking is essential. Being able to see emails, calls, donations, and other interactions in one donor record makes it much easier to maintain a personal relationship.
I would also clean up your existing data before choosing a platform. It can save considerable trouble later. Getting someone from finance involved early is sensible too, since they may notice integration issues that others overlook.