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.
I don't know if you can squeeze one these into your budget but the Forte is excellent and has a headphone and microphone amp.
My AKG headphones sound very good with this card. http://www.ncixus.com/products/?sku=35596&promoid=1016&...
i bought mine here i have no idea why it's cheaper in Canada http://www.canadacomputers.com/product_info.php?cPath=4...
This card features the best Creative X-FI chip made and all the X-FI features but without the crappy Creative drivers and support
Plus a 2yr warranty when you register.
To truly appreciate this card you need a good set of 2.1 speakers like Klipsch Promedia 2.1