i upgraded my old 4.1 (cambridge soundworks/live!) to their newer T7700/audigy2zs and can attest to a huge improvement in quality/quantity. I'm no audiophile and spending 200+ on speakers wasn't my idea of 'wise'. got the 7700's for about 80 i think, worth every penny so far.
I turned them up as high as my ears can handle for a few moments of call of duty, and they sounded crystal clear to me, more then enough bass for me as well. but again, i don't do MP3's or Movies on my rig so don't care about any of that either. for gaming its more then plenty.
but yeah, i don't think you'll get much of a difference form 5.1 to 7.1, unless the 5.1's are completely weak.
you may not 'use' the full abilities of 7.1's at the moment, but chances are you'll have your speakers for a while (if you're like me anyway) so sooner then later they'll be used in the upcoming games.