is your pc a prebuilt pc? you didn't mention motherboard brand (or i've missed it).
anyway, according to amd's specs,
http://www.amd.com/us/products/desktop/chipsets/7-series-integrated/pages/amd-785g-chipset.aspx
your motherboard should have a pcie 2.0 x16 slot. usually the top blue slot is the pcie 2.0 x16 slot. any info on pc model number or motherboard brand and/or model number would have been better.
okay, your pc should be able to run any pcie 2.1, 2.0, 3.0 gfx cards. you might need to replace your current power supply depending on it's specs/model. you also need to specify your budget and requirements (gaming, movie watching etc.).
check your specs to see if this psu and this gfx card fit your pc:
psu - corsair cx 500 v2
http://www.newegg.com/Product/Product.aspx?Item=N82E16817139027&Tpk=corsair%20cx%20500%20v2
gfx card - sapphire radeon 6850
http://www.newegg.com/Product/Product.aspx?Item=N82E16814127576