This could be fun and informative, just download this free program to calculate pi (windows based only).
http://www.benchmarkhq.ru/english.html?/be_cpu.html (scroll down to Super Pi)
Then run the program to measure the time it takes to calculate pi to 2 million digits.

Here's my times:

1: Intel 1.3GHz Celeron (5 year old Dell desktop)
512 MB RAM SDRAM PC133
Time: 5m 41s

2: AMD Turion64 ML-32 1.8GHz (Compaq Presario V5000)
1.5 GB RAM DDR-333
Time: 2m 10s

3: Intel Centrino Duo T2050 1.6GHz (Gateway MX 6930)
1.0 GB RAM DDR2-667
Time: 1m 36s