If you already own all the software you need, its cheaper to build. But if you don't, Dell gets a better deal and can give you all that stuff for way less than retail. Building is worth it completely, many Dells are hardly upgradeable
(story: my sister bought a Dell a while ago, and wanted me to add in a CD drive for her .. but the case had only one big bay, so I couldn't. Recently, I was going to put in an extra harddrive, but the mobo only has one (ONE!!) IDE channel. Who ever heard of a single IDE mobo? its almost ridiculous. no RAM upgrades possible and there are almost no PCI slots. Its insane what Dell will do to keep their prices low.)
Note, this single IDE channel is not just on the cheaper models, my mother bought a giant workstation with SCSI HD, so they only gave here one IDE channel. And it wasn't a cheap mobo, this thing had onboard wide SCSI and dual Xeon slots (only one is filled at the moment) AGP and the whole nine yards, but when I needed to add a storage HD (networking it to the other computer is too slow) I had to buy an IDE controller . . . and the case is set up in such a way that it is a nightmare to fit it and run the cable.
Just a couple things to keep in mind. If you are using Linux or already own XP, doa homebuilt. Hell, switch to Linux. Then everything software is cheaper. or free.