To build a new pc or buy?

I have built several over the course of 5 years, but was wondering if there is a general guideline on deciding to build your own or buy one? I imagine there is a point where cost would come into play and it would just make more sense to buy one then to build your own. Anyone have a good list to go buy in this decision making process?

Right now I am planning on building one. Looking at the ASUS board, with the Core i7 920 and 6gb of ram and would upgrade my video card (not sure which one yet). I would also add a new case. The rest of the items I have (hard drives, monitors, power supply and dvd drives. To me, it would make sense to build my own since I have many of the items already on hand. Is this logical thinking or are there some really nice deals out there to be had?