The digital out will send a digital signal, which can be converted to 5.1, 6.1 or even 7.1 surround. It really depends on the digital decoder you buy.
Since I've already got a set of 5.1s, I bought the Creative DDTS-100, and it's perfect. Not too expensive, great quality, comes with remote, upconverts to 6.1, lots of inputs, etc.