Performance in games is mostly determined by the power of the graphics card.
In that respect, both of those systems are pitiful.
1) Moot point; neither is suitable for more than basic slow games.
2) Both cpu's are good and ample. Intel will be better clock for clock.
3) If the motherboard has a pci-e x16 slot, then you can install a discrete graphics card. This assumes that the case has sufficient room, and that the psu has sufficient extra power.