Well those dell speakers really suck someone gave me those and I ended up giving them away too you will hear no difference between your onboard sound and a soundcard with those speakers. Now your headphones on the other hand are half decent but to get improved sound out of headphones you really need a sound card with a built in headphone amp to drive your headphones otherwise your sound quality will be pretty much no better than onboard. Now soundcards with built in headphone amps are very expensive and will run you around $179.99 at the cheapest.
Out of the cards you listed if you are dead set on getting one then the X-Fi Titanium is the best option.
You should also check out this card too.
HT | OMEGA STRIKER 7.1 Channels PCI Interface Sound Cardhttp://www.newegg.com/Product/Product.aspx?Item=N82E16829271001