There are not "DACs" for this material unless you've got some nice pro equipment in the mix (like this http://www.gracedesign.com/products/m906/m906.htm).
I'm guessing however, that you aren't setting up a mastering studio for 5.1. So you probably want to decode consumer formats like DTS 5.1, and play those back. For that, you want something known as an "AV Receiver" ("AVR") or "AV Processor" ("SSP" or "Pre/Pro").
Here's an example from Marantz:
http://us.marantz.com/us/Products/Pages/ProductDetails.aspx?CatId=AVSeparates&SubCatId=0&ProductId=AV7005
All XLR outputs, 7.1 (it can actually do 9.1 depending on configuration, but this will require using RCA connectors too), and it will decode lots of multi-channel digital streams via HDMI and S/PDIF (things like MPCM and Blu-ray audio via HDMI, things like DTS via S/PDIF).
For substantially more money you can get the "bigger brother" from Denon:
http://usa.denon.com/US/Product/Pages/ProductDetail.aspx?PCatId=AVSolutions(DenonNA)&catalog=DenonNA_US&CatId=AVSeparates(DenonNA)&Pid=AVPA1HDCIA(DenonNA)
It's got more inputs, more features, etc.
If you don't like that route (for whatever reason), there's other options. A multi-channel pre-amplifier, like this one from Parasound:
http://www.parasound.com/halo/p7.php
Alternately, a decoder box (this could be coupled with the Parasound and run as a line-level source, it also features and internal volume control though):
http://www.gefen.com/kvm/dproduct.jsp?prod_id=8202
If you want to go cheaper, there's some pre-HDMI decoder models that you can probably find on eBay still from Sony, Technics, Yamaha, and Marantz. They usually only support Dolby Digital, but a few support DTS to some extent (some also support Dolby Digital RF). All RCA outputs, just like the Gefen. Either go with a balun or DI or use the SE connections (if you're worried about noise, you can get shielded RCA cables).
Oh, one final thing to note:
Whatever you end up with, you'll need some device (somewhere in the system) that can do something called "Bass Management" - basically that's a fancy way of saying a device that can put a crossover in between the five speakers and the subwoofer, as well as ensure that the subwoofer channel present on things like DVDs is passed along correctly. The Marantz and Denon units will take this a few steps further, and auto-configure the system once you've got everything installed. The Parasound and many of the pre-HDMI decoders have manually configurable settings (the Denon and Marantz can be tweaked by hand as well, if you wanted to), I'm not sure if the Gefen decoder does this or not. I believe the Grace assumes it's handled up-stream and applies no such correction (I've not used this one quite honestly). Some computer soundcards and player machines (like DVD players) can do this processing as well.
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