1. you def want one 4 GB over two 2GB, because that leaves the possibility of upgrading your memory much more open. with only one slot taken you can easily put more memory in the other slots.
2. For dedicated graphics, you only want ATI or nvidia. don't listen to ppl saying one is better. they're both the same shit.
From the specs you listed, they both look exactly the same. only difference is sony has one slot 4GB and gateway has two slots.
My opinion: sony vaio's are cheaply made and can break easily. don't know much about gateway but i think they are good.
I would go with a comparable HP laptop or Asus laptop. Lenovo's are really good too.
Whatever you do, stay away from Dell.