A seperate graphics card has it's own memory built onto it's graphics board so generally does not use any part of your 256mb of memory module.
More importantly though is the graphics chip.
A seperate card (depending on model) can offer far superior performance because it's graphics chip is dedicated to carrying out the one graphics processing role, An integrated GPU on the other hand has multiple tasks so is classic "jack of all trades and master of none"
But don't be fooled! modern integrated motherboards do offer reasonable graphics performance, so if you are adding a seperate graphics card, going entry level may not improve graphic performance much above onboard levels. A midling card such as a Geforce 4 MX440 or ATI's Radeon 9000 will on the other hand see rather more of a improvement. Martin Replying helps further our knowledge, without comment leaves us wondering.