Oh fricks. I was away for a while and look what this thread has become! Excellent!
Anyway, to answer mayanksharma, well I must say there is quite a difference between the Megaworks 550 and the Logitech 5500 (at least I found it, and so did the "other" indian PC magazine). The bass is much much deeper and the highs are excellent. I haven't heard your sound so I cannot compare, may be as good or even better, I don't know.
But a THX certification is a seal of approval. It means that they certify that these speakers can accurately reproduce sound as is supposed to be heard in the theatres.
You may have Pro Logic II and DTS, but there is always a difference between decoders (especially those of low quality, while not saying yours is low quality). PowerDVD for instance has a software decoder for Dolby Digital and DTS as well as ProLogic II, but its not nearly as good as the hardware decoder on the 5500. The Creative Audigy's decoder is quite good though....
and I can go on and on...
(Btw, my very close friend has a speaker system, brand name "thiel" or something. Cost 27 LAKHS!!! When he turns on the speakers (along with the four Amps, the lights in the house go dim....doom 3 anyone?)