For that price, and those power constraints, I would instead suggest the 7750. It positively sips power and is more powerful than the 640. There should also be no problem using a PCIe 3.0 in a PCIe 2.0. It shouldn't be anywhere close to bottlenecking.
Thanks for the quick response. i'm sorry but u have to be a little more specific when you say 7750. i'm assuming it’s a radeon gpu? if you say it’ll do the job I’ll check one out. I know that other companies make remakes of the cards and overclock them and add memory ect.. I am definitely looking for the fastest option possible so would you have a certain card and maker in mind or do you feel any 7750 will do? Also an HDMI port is a must.