I can't believe I haven't mentioned WinBar in this blog yet. I've used this program for the last 4 years on every computer i've had. It's a pretty configurable sys meter that docks to the top of your screen.
I have it configured to show the date, CPU, RAM and HDD usage, Volume controls, Uptime, DU Meter, etc. You can choose whatever color or opacity you like. Visually it's not the prettiest by default. Running it at 75% opacity with no separators between the modules makes it look pretty good, and prevents it from standing out too much.
Once you use this for a few days, your desktop feels naked without it. And if you really need the screen space, you can click on one of the arrows on the side to make it undock and collapse to the arrow. When programming, I constantly look at the CPU and RAM usage. It's great for when you're doing stuff on the internet, because the DU meter tells you the total downloads/uploads occuring on this machine.
Download this program (its freeware). Use version 1.2.95, not the Winbar2 public preview.