1) XP will work on the Intel i family processors.
2) Which is better really depends on how old of games you are playing. If you are playing any current released games, I would go with Windows 7 64-Bit OS for 64-bit and DX11 (XP's 64-bit version was very buggy and had bad support). If you are playing old games only, they were code for XP and will run a little better on XP over Windows & (though 7 can play them in compatibility mode).
In the end, I really see no reason not to go with Windows 7 64-bit system on any new built system.