A few reasons why I wouldn't buy it:
6GB is not suited for a Phenom II system, especially if it's 3x2GB, you'll have no dual channel capability. That reduces memory throughput by half. It's also only got 1 year warranty. If you were to build your own computer from scratch, all the parts would have at least 2-3 years warranty, some, like RAM, have lifetime warranty. The GPU is also very weak for the price. It's also only got a 450W powersupply, and knowing OEM's it's likely not even suitable for that much output. The case is also tiny. Not good for airflow, not good for longevity of components, and is also a dust trap. There is also no mention of motherboard model, which means it's a very low end one. You'll also have no overclocking options and a feature stripped BIOS.
Here's what I would do with $600:
CPU:
http://www.newegg.com/Product/Produ...08&cm_re=phenom_ii_955-_-19-103-808-_-Product (same as the prebuilt)
Mobo:
http://www.newegg.com/Product/Produ...&cm_re=am3_motherboard-_-13-131-646-_-Product
RAM:
http://www.newegg.com/Product/Produ...0231416&cm_re=8gb_DDR3-_-20-231-416-_-Product
GPU:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814161338&cm_re=5770-_-14-161-338-_-Product
HDD:
http://www.newegg.com/Product/Product.aspx?Item=N82E16822152185
PSU:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817207012&cm_re=xfx_psu-_-17-207-012-_-Product
Case:
http://www.newegg.com/Product/Produ...m_re=coolermaster_case-_-11-119-195-_-Product
Total cost before shipping/rebates: $619.93 - After rebates: $569.93
What do you get over the prebuilt? The same CPU, more RAM that is also faster and more reliable, a quality motherboard that isn't feature stripped, a better graphics card, the fastest 1TB HDD on the market, a quality powersupply, and a better case. As well as warranty on the individual parts, and costing less money after the mail in rebates.
You'll only have to do a few things:
Read up on how to build the system -
http://www.computerforum.com/104641-how-build-computer-step-step-photos.html
Manually set up the RAM speed, voltage and timings (easily done)
Do the mail in rebates
Register all the parts for warranty with their respective companies (not required for all of them)
Acquire or borrow an operating system
