I build and repair computers as a part-time job, and I think that even a 1.5ghz is overkill for what you want.
Anything 1ghz or over will do you fine. Get 256mb or more memory (it's cheap right now, and the biggest increase in performance that you can get cheaply), and a 40gb hard drive, since they're so cheap now.
If someone does their whole website in flash, it usually dosn't matter how fast your processor is, only your internet connection's bandwidth.
If you can find a nice cheap Dell, go for it. Dells ARE some of the most reliable and quiet-running machines out there. I highly reccomend against Compaqs. I used to work at Compaq, as one of the tech guys who fixes all the broken Compaqs. Most flimsy, hastily made, hot-running computers I've ever seen.
Try one of the systems from http://www.tigerdirect.com or http://www.pcconnection.com they're very reliable companies, and their systems are well-priced.
