It's not just a case of what are the best speakers.
The quality of the soundcard plays an imporant role (the SoundBlaster is OK), but most important is how they sound to you.
You might like one pair of quite inexpensive speakers and be underwhelmed by an expensive alternative.
An inexpensive speaker setup backed by a good soundcard will always sound better than expensive varieties with a ?5 onboard sound chip - it's the same prinicple as hi-fi speakers needing a quality signal to perform at their best.
Some outlets (Staples for example in my area) have around a dozen speaker systems set up in a comparison area and you are able to switch from speaker set to speaker set; the differences in quality of sound is remarkable.
If it's just for games it's not too important, but for audio you need to take along some music on CD with which you are familiar and compare the various speaker setups for yourself.
My son recently bought some Logitech 5.1 speakers (cost was nearly ?300) that are quite superb, but he does have a high grade system with a top class soundcard...:-)