If you like the case you have or can't afford a whole new system,
You can get upgrades to get a little more life out of your system.

Here's the Newest and Fastest Computer to Date

I Call it the 4x4 x4 x4 x4
This thing is REALLY Fast! I gave it a page of it's own Here.

From AMD 3500+ to Intel Core 2 Duo 2.4ghz with 2gig memory.
(and i think its just looks good in these photo's too)
Jaime also went to a Dual Monitor Setup shown here

A Good budget Upgrade.

From AMD Duron 1.2ghz to Intel P4 2.8ghz with 1gig memory