Both systems will play any game at max settings (Doom 3 will be all high for the second system.)
I can play hl2 all highest with 4x AA and 8x AF with it being all over 60fps (over 100 when there is no water.) But this is with 1280 X 960.
The X2 will be more future proof, but right now games are single threaded. With the release of the next gen consoles which are multi procesors than games will start to be made for them.