10.5 will run but if your PowerBook has a clock speed of less than 867 MHz you'll need to edit a file on the DVD before it will allow you to install it - it's more of an artificial restriction to make you buy a new Mac than the machine not actually being up to running it well though. 256 is a little on the low side, though Tiger would be OK on that if you make sure you have no Dashboard widgets open and are only looking to do basic tasks. If you're looking to do more than basic tasks or would like Leopard then upgrading to 512 MB or 1 GB is highly recommended (Leopard will also block installation on less than 512 MB, so that's another line in the same file to edit if you really want to stick with 256 MB). Panther's getting a bit old now and is not really any lighter than Tiger (the only extra loads in Tiger, Spotlight and Dashboard can even be disabled completely if you so wish), particularly not on a G4 as the additional optimisations in Tiger often make it faster than Panther, especially when AltiVec is available.