You can make Linux run zippier on a slower computer by turning off font anti-aliasing. However, this means fonts as ugly as Windows...I choose to live with the performance hit font anti-aliasing comes with.

Another big thing you can do is use faster, more lightweight applications. GNOME's default file manager is big and slow. I prefer "pcmanfm". It's a fast lightweight file manager--not the fastest or the lightest, but it's the fastest I've found with the features I want (like tabbed browsing, pretty icons, graphics thumbnails, and such).

Rox filer is faster and lighter, but it's a bit limited to me and it doesn't use cut/paste the way I'm used to.

With 256megs of RAM, you should be able to run GNOME just fine, but using a faster file manager will have a huge impact on your general computing experience.