You would only need to upgrade the power supply if you decided to upgrade the videocard. If you did upgrade the videocard, that would dictate what power supply you would need.
Below is an example of a decent power supply that should work for most graphics cards. It’s $40 ($60 with a $20 rebate).
http://www.newegg.com/Product/Product.aspx?Item=N82E16817153167
How does the computer below compare to this one for just general use, some graphics work and general office work and web surfing?
HP ENVY 700-229c Desktop, Intel Core i7-4770 Quad-Core 3.4GHz, 12GB DDR3, 2TB HDD + 16GB SSD, 802.11ac 2x2 Dual-Band, Bluetooth, Win8.1 for $699.99
You can’t build a quad core with these specs for close to this price. Check out http://www.logicalincrements.com/ for specs to build at many price points.
The i7 pretty much trumps anything AMD has. That, and the power supply is big enough to handle a fairly moderate upgrade on the video card.