"nice pc... from bestbuy"
Hmm, completely contradictory.

"it will take me ages to build"
Yeah, not really. In fact, in most cases, it's much faster to order components and build a rig than it is to order a pre-built and have it delivered.

First, I'll list the issues with the linked system, and then I'll offer you a better rig if you built yourself.

Okay, ready? Good. The linked system uses an AM3+ CPU, and mid ranged one at that. AM3+ is a dead socket, over 2 years old at this point with no upgradability. It's Intel equivalent will perform far better. R7 250 is an entry level GPU at best and won't do much. PSU is guaranteed to be shit, that is bad.

For a little over $700 after rebates, if you stop being a scrub and build a PC, you could get this;
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The performance of a GTX 970 and R7 250 simply aren't comparable. The GTX 970 will blow the R7 250 out of the water. In fact, the GTX 970 is one of the best cards on the market and arguably, the best card for price:performance. 4440 significantly out-performs the FX-6300. An actual decent quality PSU that won't explode. A very nice case as well. The only downside to this build is that you'll want to add another 4GB RAM when you can, but at this point, even 4GB RAM will be fine.