Macs cost a ton.
As of right now (August 2008), the cheapest upgradable Mac on Apple's website is $2,799. On Dell's website, you can get an upgradable system for just $269. Sure, the Apple system comes with better stuff and more accessories, but what if you don't need or want all that? What if you just want a box to work on?
When I got my Power Mac G4 (Digital Audio) system in the summer of 2008, I got it for just $50.
According to EveryMac.com, my system cost $2,199 when it came out. Ouch.
Because of Macs costing so much, many Mac users try to hold on to their old equipment for as long as possible.
One of the things that helps keep a Mac usable with current software are processor upgrades.
If you're using a G3 or G4 Mac, you may be able to upgrade the processor to something several times faster than what it currently has.
Other World Computing (OWC) has a decent selection of upgrade processors.
http://eshop.macsales.com/shop/accelerators/
OWC's Mercury Extreme and Giga Designs are some of the upgrade processors people use.
According to a user on the Applefritter, the Mercury Extreme and Giga Design 7455 CPU cards use the exact same controls for VCore adjustments.
Here is the info sheet provided for the Giga Design CPU card:
(Original image from xlr8yourmac.com.)
Messing with the VCore controls will void your warranty and possibly damage the CPU. Some have altered their VCore (such as bumping it from 1.60v or 1.65v to 1.70v or 1.75v) in order to achieve a speed higher than what the CPU was rated at.
The cooling on the OWC Mercury Extreme leaves a bit to be desired.
The heatsink on it just happens to be 80mm wide, perfect size for an efficient 80mm fan!
Here is a picture of my OWC Mercury Extreme CPU card (1.4 GHz) with an 80mm fan bolted down to it:
(Click for large.)
You will need to find some long screws that fit between the "fins" on the heatsink, and are long enough to reach the heatsink through the fan itself.
Compared to the original cooling, two 40mm fans, the 80mm fan is just as quiet but moves a LOT more air.
Some upgrade cards:
PowerLogix PowerForce
Sonnet Encore
OWC Mercury Extreme
Giga Designs
Newer Technology MAXPower
Common upgrade processors:
G4 7447 (512 KB L2)
G4 7455 (256 KB L2, 2048 KB L3)
G4 7448 (1024 KB L2)
* The 7447 is usually the cheapest upgrade. It is low power, but has the least amount of cache compared to the other upgrade CPUs.
* The 7455 is a popular choice. It draws the most power, but has a large L3 cache. Clock for clock, this one is usually faster than the 7447.
* The 7448 is the newest design. It has a good amount of cache and uses less power than the 7455. It is more expensive than the others.
Wikipedia's PowerPC G4 page: http://en.wikipedia.org/wiki/PowerPC_G4