If you want a pre-built, then dirtmountain's system is much better than the one from Best Buy, but see below, for why I wouldn't buy it. If you choose to build one yourself, Jim's build is reasonable too; I won't pick nits over it, but you may not have his luck on unlocking additional cores on the 550BE, and/or they may not be stable, so don't count on that. In your place, if budget allows, I'd get a 720BE or a X4 9xx.
I do see three potential issues with the CyberPower PC. First, the PSU is an old, inefficient design and may be utter garbage; note the voltage switch on it. Considering that a quality 450W PSU would power that rig, the "700W" one in it is undoubtedly overrated. Noting that the case is Xion, the PSU probably is this one, reviewed at jonnyguru: http://www.jonnyguru.com/modules.php?name=NDReviews&op=Story&reid=109. Yup, it's crap.
Issue #2 is the non-expandable micro-ATX board in it; all RAM slots are filled, and future SLI or Crossfire will not be possible.
Issue #3 is minor in comparison, but the cooling in that case is not good, with only an 80mm fan in the rear. This is not a system with any serious overclocking potential. It also has the stock Intel cooler on it.