well... 2 things... ...im assuming you want surround sound, as you have a receiver with 5.1 channels that youre wanting to use... and you want a card that offers an optical output... my question is, do you want to be able to get surround via that optical output?, or would you be comfortable just using analog cabling instead?
if you want surround over optical for general usage (music, games, etc), you have to get a sound card that offers dolby digital live, or dts interactive... both of those formats output 5.1 for everything... but, creatives cards do not offer either of those features... without either of those formats, youre stuck getting only 2 channels over the optical connection, as a pcm signal... not good for surround at any rate.
a large exception to that, is if you want to watch a dvd on your computer... with dvds you can get the same surround offered by the ddl and dts interactive cards (the signal is output as an ac-3 or dts signal then, not pcm)... and any sound card with an optical output will be able to have surround sound then (for the dvd anyhow)
if youre comfortable just using analog outputs for surround, most sound cards can do that, so, your options are much more abundant then
the only current card that offers both ddl and dts is the Auzentech HDA X-plosion (its also ~$130 at newegg)... the other cards that offer only ddl are the Turtle Beach Montego DDL (~$80), Blue Gears HDA X-Mystique (~$100)... theres also the Auzentech HDA X-meridian, which isnt out yet, but, its supposed to offer ddl, dts, and superb analog output as well
creative also offered the DTS-610 interconnect, i dont know if they still do or not, but it would take 6 seperate analog cables from your sound card (5.1), and convert those analog channels into a dts stream using an external converter, essentially doing the same thing as the above sound cards do, but also being able to bypass DRMs from what i remember, as a side benefit (as the sound is output from the card entirely as analog, thus removing the encrypted data, and then converting the analog wavelengths to digital), but, it was ~$100 when i saw it before... ...a downside of it that comes to mind though, is that your sound card should preferrably have decent quality analog output, to minimize the effects of the dts compression (making the effects less noticable anyhow)... so, in that respect, the dts-610 is the least affordable option, needing both a good analog sound card, and the interconnect for things to work well