I am upgrading my laptop with a 1gb of memory. At the moment there are 2 - 256mb memory cards in. Should I leave one of these in and would I get 1.256gb of memory. The 1gb memory is 333mhz and the 256mb is 266mhz. Is this a problem.

You may run into any of two problems:
- the BIOS may not support ("see") that amount of RAM
- the speed differential of the RAM

I can't answer these questions, you may be better off on the Dell forums.

Since you have the memory available to you, why don't you just do some testing?

Post back here with results so we can all learn something from this!