I agree, you shouldn't need much horsepower if the server is dedicated to SageTV. If you have an old system, use whatever CPU you have on hand. If you're buying new, get whatever falls in your price sweet-spot. I ended up with an Athlon 64 X2 4200+. I haven't put Sage on it yet, but it's a much more powerful CPU than my current Sage system, which records 4 tuners and does commercial-skip processing.
Tim