What type (form factor) Power Supply Do I need?

Hi, I'm building my first computer and I'm confused about what size of Power Supply to put in my build. I am using the Antec Nine Hundred Mid-Tower Case, and I need a 500-700 watt PSU. Are the ATX and ATX 12V the same physical size or are they made to fit different cases. I thought the default was ATX for mid-tower cases, but on Newegg.com, where I'm ordering my parts from, the majority of the power supplies are ATX 12V. What exactly does this mean? Will either an ATX or ATX 12V fit inside of the Antec Nine Hundred?

I'm ordering my PC tomorrow, but I'm really confused about this one issue. This is the PSU that I'm looking at right now, because I was under the impression that I needed a normal ATX power supply, but I'm not sure:http://www.newegg.com/Product/Product.aspx?Item=N8...

Will that PSU fit, or do I need an ATX 12V???

Also, In case it matters, here are my specs:

250gb Seagate Barracuda HDD

Dual nVidia Geforce 8600gt 512mb ddr2

amd phenom X4 9600

4 gb ddr2 1066mhz ram

Please Help! I'm really confused!