A cheapie SunFire v200/210 will run like a tank, but you'll be crippled by the server's top speed, and they do put out the heat if you push up the load average (and HVAC costs should always be factored in, yo.)
You'll also need to buy a lot of those pizza boxes to make up for the processing power that you can find in a box half its age, let alone the newer iron.
Sometimes you have to run the old stuff (I work in an environment where we have testbed boxes, and SunFires are a part of that, along with ancient RS/6000 gear, PA-RISC HPUX gear, etc. I can tell you right now that the old stuff cranks out a lot more heat (and in many cases eats a lot more rackspace) than the equivalent horsepower found in just a handful of new HP DL-360's.