that's how you'll have a stereo output (front) of your mix to your amp and a different preview output (rear) to your headphones. the units you mentioned only have mic (input)/headphone (output) jacks. whatever you get will replace your laptop audio.
since you have a desktop, any $30 internal sound card would be perfect and guaranteed to work.
diamond xs 7.1, $34: http://www.newegg.com/Product/Product.aspx?Item=N82E16829111001
audigy se 7.1, $28: http://www.newegg.com/Product/Product.aspx?Item=N82E16829102003
since you want to use usb, you might have problems because the vendor provided drivers are often crappy. this gwc should work for you (but isn't saving you any money).
gwc 5.1, $27: http://www.newegg.com/Product/Product.aspx?Item=N82E16829126101
this x-fi will definitely work with virtual dj (see first user comment).
audigy x-fi 5.1 usb, $68: http://www.newegg.com/Product/Product.aspx?Item=N82E16829102020
I'm not too sure what I'm doing but I know I need a sound card. While using Virtual dj, I want to preview songs separately in headphones, while my speakers continue playing my mix (totally different output), so will an external usb audio adapter like this work?
(cheap cause I have no money) or something like this?
It apparently needs to be at least 4.1 channel surround to work great.. Will those ^^ ^^ work alright?
OR will I need something big like this, with Digital i/o ?
BTW I'm using a DELL desktop and I don't want an "internal" sound card and I have my speakers hooked up to the back so I can use my USB in the front..
and I won't be using a mic, I only need an audio OUTPUT for my headphones while my speakers continue playing something else..
UGH I'm so lost