Alienware X51 with an i3770K configution, 8GB of RAM and 1TB hard drive is about $1k
http://www.dell.com/us/p/alienware-x51/pd
Next to Xbox 360 and PS3 Fat
Next to Xbox 360 S.
It has an external PSU similar to the 360
Review: http://www.theverge.com/2012/2/3/2768359/alienware-x51-review
Alienware X51: Graphics card upgrade discussion
http://en.community.dell.com/owners-club/alienware/f/3746/t/19442269.aspx?PageIndex=100
So you can just buy the basic version with i7 3770K and then throw in a separate GTX670 videocard and an SSD of your choice (Samsung 840 Pro or OCZ Vector 256GB) in there. That would destroy a laptop but of course with such a small case/form factor, your future upgrade path for PC parts would be a lot more compromised.
GTX670 can be bought for $310 now: http://www.newegg.com/Product/Product.aspx?Item=N82E16814133452
