The only thing I disagree with Lex is the cost of Vista. You can get legitimate copies of Vista Business for $140 or Home Premium for $100-110. They are OEM copies, but completely legitimate and they are the exact same copies that Dell or HP would use.
The only way I would recommend building the PC's is if you are warrantying these yourself, and are prepared for any costs associated with repairs. Also, you would be responsible for any technical support related to Vista since you would be the OEM provider. That is another advantage of a Dell or HP in this situation. You get the warranty, repairs, and Tech Support taken care of.