1.33 Athalon - Safe Operating Temperature?

I rebuilt a system on a new FIC AU-13 motherboard, reusing my older Athalon
1.33 CPU. I purchased this OEM chip 3 years ago along with an AMD -
recommended Coolermaster heat-sink & fan. When I did the rebuild, I lapped
the large aluminum heat-sink to a fine finish, using 2000-grit sandpaper and
cleaned the CPU die with isopropyl alcohol. I finished by applying a fine
coat of Arctic Silver 5 thermal paste, per the instructions on their
website. I installed in an ATX mid-tower case with a single 80 mm case fan
(blowing in).

Despite this effort, CPU temps range from 47 Celsius at ambient room
temperatures in the low 70's (Fahrenheit) to 55 Celsius at room temperatures
in the low 80's (Fahrenheit). These temps occur after 15 or 20 minutes of
light operation (web surfing). I fear these temps are too high for sustained
system health. I had hoped to achieve a modest overclock with this CPU but
with these temperatures, I won't even try.

Questions:

1. Am I correct in assuming that these temps are too high?
2. Can someone recommend an affordable heat-sink/fan combo that will
significantly reduce the CPU temperature? My budget is tight ($20 - $25 US).

Thanks for any wisdom or input.