From hps site it shows it came with a 1.86ghz core 2 duo and 1gb ram, and well thats just not near enough to run also any game like u mentioned, but u didnt give specs but heres what i got. (Specs r nice to give lol)
well max ram u can throw onto the board if which is 4gb 667mhz ddr2, and upgrade to a higher clocked core 2 duo from the e6x00/e4x000, and a new gpu something that'll play bf3 cod skyrim etc like a 7770/7790/7850, 650/ti/ti boost/ 660gtx and a new power supply.
But to buy a new gpu and a psu and also buy more ram if u need too and a higher clocked core 2 duo u might as well build all completely new and keep what u can like the hdd or disc drive.
Personally if u need a new pc i would get a amd A10 apu with no gpu added until u feel u need one, a decent board like a asus/gigabyte/asrock A75/a85x board, 8gb ddr3 1866, a corsair cx430, a wd 500gb/1tb hdd, new case, and whatever else u need like disc drive, peripherals, OS, etc. The hardware mentioned at the momment being exactly a a10 5800k, asrock extreme 6 fm2, corsair cx430, wd 1tb blue, 8gb 1866 ddr3, and a $60 case will run a little under $500 with no os, odd, gpu, wifi, etc