You should be fine, but why not build your PC? Its a great experience and if anything really breaks or happens to it hardware wise you could probably fix it on your own. Not to mention that it would save you around 200 bucks to build it on your own.
The computer you are going to buy is junk. Always build your own computer, you will get better performance and parts then prebuild computers. That computer there is only worth about $600 dollars. It's not hard to build it yourself. It takes time, some reading and patience. If you plaing on building a computer and you know someone who has done some work in the field, ask them to come over and help you out with the build. You will feel alot better then buying a prebuilt computer.
I think labelling something running a Phenom II and a decent ATI card "junk" is a bit over the top. But I do understand what the guy is trying to say. That computer is good, perhaps you could use it as a stepping-stone to building as many people have done. This forum has a lot of in-depth knowledge if you want to build (and look for it), but I understand that some people cannot be bothered to tinker.
But if you have the money, buy it you will not be disappointed and you can overclock if the motherboard supports it.
This is a hardware enthusiast forum so yer gonna have some people tell you to build your own PC. Just ignore them if you do not have the time / knowledge to do so. Or if you simply do not have the desire.
See following review of the HD 5770 for benchmarks. Your own benchmarks will be a little lower because the testbed PC uses an Intel Core i7 Extreme i7-975 3.33GHz which costs more than your entire PC.
You should totally build a PC, heck I wouldn't buy a iBuyPower without opening that case up and makin sure everything was ok, who knows what they do. Probably just slap it all together and say done I have heard stories though where people who bought pre-built machines had a graphics card not work or something and were like "OMG its broken " and it just so happened that the 6pin power cable wasn't connected to their card, but they ended up sending there PC back anyways wasting a fair amount of cash on shipping. Because they had no idea how the heck their PC worked anyways.