I run an Athlon 2.2 GHz machine - only difference versus the older Intel machines is the chip needs more cooling. I pulled the case off to give it better circulation, and the fan noise is rather more than I was used to. If you don't mind having a wind turbine under your desk, then Athlon is fine.
You do know that having the case open is generally *worse* for cooling, right? I've got an Athlon XP 1800+ which, while it has more cooling fans, is quieter than the P3-500 it replaced. It's quieter because the fans are all larger, lower rpm fans. Even under heavy loads, CPU temp rarely goes above 39C. I ran it with the case open for a while becuase I hadn't finished moving parts into it, and the temp was usually around 45C. Most (decent) cases nowadays, are designed so that air flows nicely through the case to draw in cool air and push out the hot.. My case has intake fans in the front and side (over the video card) and outgoing fans in the back and the power supply (yes, air flows *through* the psu). -- trey