What do I need to consider when buying a digital camera?

The XSi is a digital camera, perhaps you've confused it with another model. If you like Canon you should stick with them, you'll already have lenses, and be familiar with the system. For $500-$700 you could pick used 40D, or maybe a new T2i. On the Nikon side, you could get D3100 and kit lens, or used D90 for about that price. When comparing brands you should think about what you are going to be shooting. Which system is going to offer the best selection of lenses? When you want to upgrade, which system has what your looking for? If you shoot landscapes, Canon's 5Dmk ii has FF sensor and high MP count. Nikon's only competitor is the D3x($8000). If you shoot sports, Nikon has the D3s, FF sensor and 9fps. Browse around on Amazon, look at prices and reviews.