how well do you think these creative's would compare to other mid-price 5.1 speakers, like logitech's z-640's?

these are the lowest-end 5.1 speakers i know of, so z-640's i expect would perform better

I have a set of 640s at my desk at work and I think they are pretty kickass. I listen to di.fm (Kurtis' fault) or accuradio.com and the music comes through pretty well. The crossover seems to do a good job of keeping the bass out of the satellites and the bass is strong enough for the low bitrate stream im getting. MP3's come through pretty well too. And since the 640s are about $50 online, its a great deal.

Of course, I havent heard the 5400s so can't compare sound quality.

It was very useful for me