I've added a couple SSDs, more RAM and a new video card. It runs a 27" Eizo beautifully and the entire thing is a stable computing money maker.
Waiting (seems for ever) for Apple to update their Mac Pros, in the meantime I have been looking at various video cards for my Mac Pro (2x 2.8 GHz quad - 2008 - 16 GB RAM). The stock ATI Radeon HD 2600 failed and was replaced by Apple under warranty. The HD 2600 is sometimes flakey (only very slightly) driving Photoshop CS5 on dual monitors - Eizo 241 with NEC 21" (tools palette).
Which video card did you upgrade to, please?