Graphic card upgrade

Wil work because PCIE 2.0 , 2.1 are backward compatibles.
Power supply CORSAIR Builder Series CX500 http://www.newegg.com/Product/Product.aspx?Item=N82E168...

You should be fine. The Radeon cards work with the P4x chipsets.

What PSU do you have now? The 6670 doesn't use a lot of power so you might not need a new PSU.

i think u have this psu General 450 Watt PSU this is ok for now but it will be good to get a new psu which will be more future proof but get that after some time not now. 

Yes with BIOS updates (if needed) should work. As I tell you if you are planning to get CX600 go get it. It'll be good for you and have good upgradebility. Is it 6770 or 6670? If I'm not wrong you said 6670 before. Both 6670 or 6770 will do in your 450W (If it's lower than 24A on the +12V Rail No for 6770 sorry, but mostly 450W will have above 24A). But if you are going to get CX600, it is not wrong idea. Go get it.

no man he said hd 6770 in his previous thread.