Yes, you can't switch out the (so-called) video card.
But, to add to blackout8, 256 MB over 128 MB indeed wouldn't have a great performance advantage but it would last longer. That is, newer games would need more and more memory for more and more textures that are more and more detailed and whatnot. Not last longer as in would die later on. Getting 256 MB over 128 MB would be looking into the future. Then again, my PC's 4-year old GeForce 4 has 128 MB of VRAM. And then yet again, it hasn't failed me yet. Runs great with even newer games. Some games only at lower res (FEAR), and one doesn't work at all (Oblivion--needs newer shader versions), but HL2 works amazingly great. But, I don't really play video games. Maybe some older ones.