Will the HD6670 work with my computer?

I'm currently shopping around for GPUs for my pre-built HP computer. I've had other posters on the site recommend me the HD7750, but right now I have my eyes on the HD6670 mainly cause of the cost. The main reason I'm making the upgrade is so that I can run Diablo 3 on medium settings smoothly. My budget for the card is under $100, so I'm still debating between the two cards. I'm not sure how much performance I am sacrificing over cost. I also want to upgrade the video card without replacing my PSU. The specs for my computer are:

AMD Athlon™ II X4 quad-core processor 620 2.6GHz

8GB PC2-6400 DDR2

1TB Serial ATA hard drive (5400 rpm)

NVIDIA GeForce 9100 graphics Features 256MB integrated shared graphics memory

with a 300 watt psu.

http://h10025.www1.hp.com/ewfrf/wc/document?lc=en&dlc=e...

This is the graphic card I am interested in: http://www.newegg.com/Product/Product.aspx?Item=N82E168...

This was the card recommended to me earlier: http://www.newegg.com/Product/Product.aspx?Item=N82E168...

If it is not any trouble, can anyone compare the two cards and check if they will fit in the stock mobo of my comp? I'm also not sure if these cards would work since the newegg psu calculator said it needs 308 watts for the 7750 and 322 watts for the 6670. Are there anything wrong with the brands of the cards I picked by the way? Any advice would be greatly appreciated.