[PSU Comparison]

Hello I'm pretty new in the computer hardware area, but I would like to upgrade my pre-built computer with a new PSU to support a new Graphics Card which I haven't even picked out yet.

This is my computer:
http://h10025.www1.hp.com/ewfrf/wc/document?docname=c02...

And these are the two PSU's I would like to know about.
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

http://www.newegg.com/Product/Product.aspx?Item=N82E168...

The first one is obviously more expensive and bigger as it looks, and I want to know what's so different about it and if it is better than the second one because I don't want to just waste money on something that's not worth it.
Also, the first one is bigger in size, will that fit my computer or do I need a special case for that?

Thanks