In my opinion I don't think you'll be wasting your money - beyond the fact that 139.00 is pretty darn inexpensive, here's my reasoning...
Starting with a 5.1 setup, you can either connect the card's S/PDIF to your receiver's S/PDIF-in or, connect the cards analog Front Left, FR, C, S, SL, and SR to your receiver's 5.1 analog input. Note that if you are using the digital connection you aren't using the soundcard's dac, nor will be able to play dvd-a once that option is available (which as I understand it, requires using the card's analog outputs).
Now let's say you add powered rear surround speakers for full 7.1...
Feed the card's FL,FR, C, S, SL, SR to your receiver as before.
Feed the card's Rear L, Rear R to your new powered rear surround speakers.
(you'll probably need to tell windows and maybe the card's control panel your new speaker setup, and you'll need a software dvd movie player that can decode to 7.1).
The only disadvantage with this arrangement that comes to my mind is that if you prefer to have the source (which is the soundcard) set at maximum volume and control the volume via hardware, you will have to control it at 2 places - the receiver and the powered speaker system volume control.
Edit - I should probably add that many recommend having similar speakers all the way around for balanced home theater sound etc... So if you are aiming for the full-on audiophile setup, just ignore me

But I personally think you could achieve a respectable sound with something like I described above (just an opinion).