Dell Optiplex Gx620 video card help

I recently acquired a Dell Optiplex GX620 sff (small form factor) computer for free, and i was wondering if i could turn it into a mid-range gaming computer. I realize that the computer is old and out of date but i want to know if i can put in either this video card

http://www.mightyape.co.nz/product/Sapphire-Radeon-HD65...

(Radeon HD6570)

or this card http://www.mightyape.co.nz/product/Asus-HD-6450-1GB-GDD...

(Radeon HD 6450 low profile)

The computer only has 245 Watts but i have heard that the 6450 a very small amount of power?

Any help would be greatly appreciated.