I've run it on a 1.5Ghz 12" PBG4 with the max 1.25GB of RAM and it was perfectly acceptable. Depending on your tastes, I recommend at least 1GHZ and 1GB of RAM to run Leopard (from it's 866MHZ, 512MB requirements).
If you don't have a need for Leopard, then there is no reason to upgrade as Tiger will run better, and is still a great OS.