ViDER

yeah, a good reciever should work. I'm not up-to-par with the xBox's capabilities, though, in regards to Dolby encoding . . . but, still, you don't need Dolby to make use of a good reciever. a highly quality reciever should be able to properly mix the channel output . . .

Regarding an EAX capable card - TBH, if you're going to go with a full-blown reciever setup, with optical or digital SPDIF connections . . . I'd recommend an Auzen X-Fi. Audio quality is much better than 95% of Creative's line up, you'll have EAX 5.0HD support, as well as Dolby encoding and decoding . . . plus Creative's proprietary software features, like CMSS-3D (which is the best speaker virtualization solution I've heard) which can make even 5.1 or 7.1 setups sound completely amazing. The cards aren't as fast as Creative's X-Fi equipped cards, but in today's age, the slight difference between the two isn't noticeable.

Auzens are a bit more expensive, but if you're going to have a killer setup, a killer quality card is defi worth it's value

Either way, skulking about The City will never sound better!

personally - EAX on PC.

From what all I've read, the xBox version stays very true to the series, but for those of us who were big into Thief back when The Dark Project released . . . there's just something about the more intricate control available on the PC . . . as well, with modern video hardware, (IMHO) visually it's much better, too.

But, either way, the game is excellent on both platform (and you'll love the Shalebridge Cradle mission ).

Regarding EAX - Thief is one of the few games where audio was a big deal during development, so EAX wasn't exactly slapped on at the last minute (even for Thief 1 and 2, the EAX "patch" spent a good amount of time in development).
Asides, Thief 3 is one of the few games that supports EAX 4HD, and the environmental effects are simply amazing . . . and, unless I'm mistaken, you can run EAX in-game through Dolby encoding to a good reciever . . . I don't believe that you have to choose one or the other (depending on the audio card).