Is it possible for me to upgrade my graphic card on Optiplex GX620 SSF?

My specs are

Intel Pentium D CPU 3.00GHz 2.99GHZ
RAM 2.00GB
Intel 82945g chipset family. I don't know if that needed

What i found is that upgrading my card to AMD Radeon HD 6570 or 5450 is way better and it will let me play my basic games like Garry's Mod, Minecraft and Dead 4 life 2

does that mean i need to upgrade my CPU or no?

My power supply is 275 wat - do i have to upgrade this?

i just want to upgrade so i can play my favorite games

please help me here D: i'm clueless

my desktop is Dell small factor (GX620)

akod

First, upgrade that ram to atleast have 4 or 8gb total. You don't have to upgrade your power supply if your willing to get this: http://www.newegg.com/...6814161418
Other graphics cards require more power and will require a better supply around 400 W

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