(Redirected) GPU that will work with my dell optiplex 780

Hi. I have recently bought a dell optiplex 780 desktop computer (DT). I want to upgrade the GPU but I do not know what options I have or what I can get. I mainly play minecraft and MMORPG games. Does anyone have any suggestions of what I may be able to get without breaking the bank or having to buy a new power supply? If you could help, i would really appreciate it. thanks again.

specs:

CPU: Intel Core2Duo E7600 (3.06Ghz)

RAM: 4GB (I dont know if its DDR2 or DDR3)

GPU: Integrated (believe it an Intel GMA 4500)

OS: windows 7 (64-bit)

link to pdf file of dell optiplex 780 (the one i have is the Dell Optiplex 780 desktop computer (DT)

www.dell.com/.../optiplex_780_tech_guidebook_en.pdf