what kind of CPU power and RAM do you have behind that video card? (Judging by your name) If you have a P3 700 or better, soft decoding should be fine. I have a Duron 700 with 128 MB of RAM and it'll play DVDs very smooth (almost an unnoticable difference from hardware decoding). A good DVD decoder card (Hollywood or Creative) is going to run you about $50 so if you have the CPU/RAM power I would just go with the $6 soft decoder, for now at least.
I think it'll be very close.....I think PowerDVD and WinDVD have demo versions for their software. Seems to me PowerDVD would allow you to download and install.....but would only play 5 or 10 mins of the movie before shutting down. Should be enough to see if you can go without a hardware decoder.