the i5-450 is a solid processor, and gaming performance difference when compared to the i7 with the same hardware is going to be minimal.
Oh, and all of those brands are good. IMO sony is going to be a bit more high end oriented (with a price to match typically). Asus is a good general brand and lenovo tends to be business oriented. Lastly, Dell is a good consumer brand too with more customization options than other brands (although some people seem to really be distrustful of dell, probably because of bad experiences, but i personally trust them)