i think it would be better for your budget range if you went with a phenom II x6 and amd 6850 and added a solid state drive as your main os/program drive. This would show a bigger boot time cut and speed increase over any system without a ssd. but thats just me.
I would agree with the AMD build. Just built one very similar only with ASUS mobo
, Antec 300 case, and Antec 750w PSU. It is a solid gaming rig that should last for a while. Nothing against Intel (I have an Intel build also) but right now price for performance is AMD IMO. I would consider the Rosewill Blackbone Case as I just did a build for my son in one and was very impressed with it's quality for the $29 (on sale - regular $39) spent on it.
Actually the AMD build with the CoolerMaster Haf912 case above looks awesome, the only things i would change (mind you this is just personal preference for the most part) is the video card and power supply. I would change both of these in case i wanted to do Crossfire down the road,, which you said you did. I would pay the extra 10 bucks to go with XFX and get the double lifetime warranty they offer. I dont see why anyone wouldnt do that. Also i would go with the 700 Watt PSU so you could effectivly run two video cards. Here is a graph of the wattage the card can pull idle and at full load. Tack this onto a 125 Watt proc and you will need at least 700 if you ever have 2 cards.