There isn't really much for the Mac in the way of a middle ground of a very versatile, good quality soundcard with some genuine benefits for the regular joe like the Creative X-fi.
First of all you've got the built-in sound similar to many other laptops which feature mediocre sound, which for some reason magically apparently becomes a super high fidelity device among the Mac enthusiasts when it turns up in a Macbook Pro. One of the things you can do to make the most of the onboard sound is to use a digital set of speakers. Outputting a digital signal makes the most of the average sound on the Macbook Pro.
I don't know of the speakers you already have are capable of optical digital input. If they are, you're set.
The other option is semi-pro to pro soundcards. There's a huge range on offer there, but any of these might be overkill for what you want. I'd avoid the low-cost Mac audio solutions, unless, like the Griffin Firewave, you need 5.1 analog output.
If your speakers are analog only, rather than buying something like the Firewave I'd say take a look at Logitech's digital-input speakers, and plug your MBP in directly in via optical to these. This would be the most cost effective way of getting reasonable sound upgrade and control of that sound out of the Macbook Pro.