Sbox is really resource-friendly, it will eat as much or as little of the bandwidth as you want to feed it. But for local resources its really light-weight.
That said, dude, 256MB RAM with up to 64 of that being shared video? Upgrading your RAM even just to 512MB (preferrably more) will make your life better on a number of levels (above and beyond just sbox - kick it up to 512 or better 1GB and you've got a pretty nice little machine).