Here's an example of a $700+ prebuilt PC:
http://www.newegg.com/Product/Product.aspx?Item=N82E16883227391
Purchasing a pre-built computer for gaming is usually a bad idea and that one is no exception. They work for a while but companies like iBUYPOWER and CyberpowerPC always come with low-quality PSUs that no experienced builder should ever consider purchasing individually. If it's made of parts you wouldn't buy on their own, then it's not a good idea to buy it all put together

Building your own PC can be challenging but is very rewarding. There is a huge world of information on the internet out there to assist you with building, including individuals like myself. For my first build, I watched Newgg's tutorial videos and learned a lot from them. It's a three-part series that covers all the basics.
http://www.youtube.com/watch?v=lPIXAtNGGCw&feature=player_embedded
Also, wattage recommendations from video card vendors are greatly exaggerated to protect themselves from liability with the many junk PSUs out there. 40% over the calculated reality wattage consumption of the entire system is more on the lines of what I'd recommend.
The problem with that logic is the assumption that even on good-quality PSUs that the rated wattage output labeled on the unit realistically represents the wattage capabilities you can expect after several months of gaming. PSUs are tested and rated in near-perfect lab conditions, and will not account for the extra heat, dust, your particular outlet's own irregularities, and any irregular quirks in your particular build's power consumption after several months of gaming. PSUs will deteriorate over time just like any machine, and the harder you push your PSU the more quickly that will happen. This is why overshooting the wattage significantly, even when selecting a high quality unit such as a Corsair TX or SeaSonic, is important and why you can never really have "too much wattage." You're only purchasing more reliability.
How much you want to spend on reliability for your $700 or more new computer is at your discretion, but I will advise what I've learned to be the safest guidelines.
While I myself am very active in suggesting to never overlook the PSU, if the member is already running a Radeon 5850 comfortably...think its rather safe to assume that the 7850 would be fine with the member's PSU. Plus it consumes less power than the Radeon 5850.
His PSU wasn't stated, but it's possible he could have a dying Enermax in there just waiting to go bad on him. And in fact the longer a low-quality unit is being used, the more concern I would have for its reliability.