New computer build

[Roark]

BTW: I am really used to Windows XP. If I will stay with XP on the new computer, will I see difference in the performance?

Depends on the games you are playing.
As far as I know Windows XP is limited to DirectX 9, and a lot of the newer games use DX 10 or 11.

I used XP up until recently myself, then switched to Windows 7.
Honestly, its not a hard move to make.