Thanks man thats a lot of help so if i got a sub with 4 ohm dvc id have to wire one bridged channel to one side and the other bridged channel to the other side of the sub?
Noooo don't do that unless you're running a mono signal to both sides of that amp from the head unit. Running left and right signals to the same sub on different voice coils will fry things pretty fast. If you run a single sub I'd really recommend something besides a 4 channel amp, I've never really seen that work out right. You can get by with a 2 channel bridged on a single sub. Rockford Fosgate's wiring diagrams are a decent tool for reference if you're not too familiar with impedance, etc.
http://www.rockfordfosgate.com/rftech/woofer_wizard.asp
Can you give us the make and model of your amp? Those power specs seem a little strange to me. Not sure how they went from 75x4 to 240x2 unless they rated those at different impedance.
EDIT - Also as far as the impedance ratings (Ohms) you should really pick what subs you need based on what output impedance your amp is capable of and how you're going to wire everything up. For instance, a single sub with two 2Ohm voice coils can be wired up to present a 2Ohm load or an 8Ohm load depending on how you hook everything up and what your amp is capable of. Dual 4Ohm coils have different possible combinations, and combining multiple woofers of the same type can give you even more options. Lower impedance generally means more power when the amp stays the same, but if you wire your subs up to present a 2Ohm load and your amp is only capable of a 4Ohm load, you're going to fry that amp.
Good luck! Designing the stereo in my reg. cab was one of the most fun projects I've ever done.