Help upgrading HP p6720f

[nhenn]

I've got the factory model HP Pavillion p6720f Desktop and I am looking to upgrade it specifically so I can play newer games like Guild Wars 2 on medium-high settings. I know I need to upgrade my power supply (since it's only 250W output) but what graphics card would be best for achieving my goal? I've looked at so many reference sites comparing cards and prices, but it is all too foreign to me. Also, what else would I even need to be able to support the graphics card recommended?

 

[Maziar]

Your board has a PCI-E x16 slot so you can use pretty much any card on it(but some big cards such as GTX 680 may have trouble fitting in),but as you said,the PSU needs to be upgraded as well.
How much do you want to spend and what resolution do you play on ?