Get onto ebay and find a used first generation i7 with say an 840QM along with a strong graphics card (GTX 560m / Radeon Mobililty HD 5870). You can find some excellent deals and the configuration of at least an 840QM/HD Mobility 5870 is still very powerful and can play most any game at decent settings. Minecraft for the PC was released in 2009. As of Jan 2010, the HD Mobiliity 5870 was the fastest laptop GPU available excluding SLI or crossfire cards. Don't discount older hardware. I only just upgraded from my first generation i7 965 EE which is still ranked by some as one of the top 10 gaming CPUs going. I have an iBuypower Battillion 101 with this configuration for around $330. It was overheating as the liquid silver was dust, plus I upgraded a 720qm for the 840QM. I use it for gaming on the road and with a Windows Experience Index of 7.1 it is a suitable desktop replacement. At best buy you would have to spend a hell of a lot more than $400 to get the same performance, if they have anything comparable in stock.
Good luck.