The Spore system requirements have been posted at the completely overhauled Spore.com, and they turn out to be surprisingly light! These are of course minimum system requirements, so for the “total experience” you may need a tad more, but still.

Windows XP
2.0 GHz P4 processor or equivalent
512 MB RAM
A 128 MB Video Card, with support for Pixel Shader 2.0
At least 6 GB of hard drive space

Windows Vista
2.0 GHz P4 processor or equivalent
768 MB RAM
A 128 MB Video Card, with support for Pixel Shader 2.0
At least 6 GB of hard drive space

Mac OS X
Mac OS X 10.5.3 Leopard or higher
Intel Core Duo Processor
1024 MB RAM
ATI X1600 or NVidia 7300 GT with 128 MB of Video RAM, or Intel Integrated GMA X3100
At least 4.7GB of hard drive space for installation, plus additional space for creations.

A complete list of supported graphics schips can be found on the forums.