I Upgraded my 3 three year old MSI motherboard from an Intel 1.80GHz Celeron to an Intel P4 2.40GHz CPU today. I went from a 128k 3640 Bogomips machine to a 512K cache 4829 bogomips one.
For more on bogomips, go here http://tldp.org/HOWTO/BogoMips/bogo-faq.html
The cost was $65 + $15 for the new CPU and fan. I get a 40% performance boost for $80 without the hassle of having to build a new machine. It took about 15 minutes for the upgrade. 10 minutes were wasted on waiting for the old CPU/Heatsink to cool down (Yes I burned my fingers).
Other than me not getting the ethernet cable plugged in all the way, no xUbuntu problems (sudo dhclient3). I haven't rebooted into Windows yet, so no telling if I'm a thief and violated my good standing with WGA by changing my CPU.
Glest loads faster but is still sluggish when I want to attack. http://www.glest.org/en/index.html
WoP sucked before but now is great http://www.moddb.com/games/350/world-of-padman
OpenOffice loads in 4 seconds where it took around 15 seconds before.
OpenArena still plays like OpenArena.
All the weird pauses in sauerbraten are gone. http://www.sauerbraten.org/
I never had any slowness problems with building programs/burning CDs/browsing at the same time on the old CPU so I won't benchmark those activities.