Nvidia ION is a built in graphics processor, aka. gpu. It may affect your system memory and thereby your random access memory, aka RAM.
As for your gfx card purchase. I would not advise playing with the ION in sli. Instead, buy a decent graphics card (almost every single one of them nowdays uses PCI-Express) or even buy two or three of them and run them in sli and then disable the ION so it wont use ur machines resources unnecessarily.
you're going to be processor limited no matter which GPU you buy. I wouldn't recommend anything beyond maybe a GT 240. Anything better would perform horribly no matter what, and would be a waste of money.
I can see it mentions how CPU can hamper the GPU...as it cannot keep up with certain tasks. But where do you draw the line. How did you know to say a GT240 should be the limit...where does the logic come in?
The Atom cpu, which you have with that ION system, is extremely slow. it is cheap and low power for the target devices it is used in, but it was never meant for gaming at all. Sure, it could play some older games, but I wouldn't play anything more than a really basic game on it.
Figure that 1.6GHz ION is about equal to a 1.2GHz Core 2. Even overclocked to 2.1GHz, it wouldn't even be up to matching a 1.6GHz Core 2. It is dual core, so you would have that going for you, but that's not much.