The general rule is simple: optical S/PDIF, aka Toslink, can ONLY carry multichannel audio when encoded in Dolby Digital or DTS. (Well, technically it could carry more, but the standard prescribes DD/DTS, so no device will be able to do anything with the signal should you be able in sending it across the cable.)
Both DVD-A and SACD have NOT standardised on DD/DTS or their lossless, high resolution counterparts. This means that for both mediums you'll only get 2 channel output. And to be honest, I'm not entirely sure how S/PDIF handles 'non-standard' sample rates and/or 24 bits bit depth. I think it depends on the device sending the signal and the drivers used. It may get resampled to 44100 or 48000Hz or it could be transferred as-is.
I'm not really sure it's worth it, with S/PDIF. With SACD you won't be able to play the DSD layer, but only the backward compatible Redbook PCM layer (basically just the CD version). With DVD-A you CAN play the DVD-A part, but like I said only in stereo. You might just as well go with the backward compatible DVD-Video part, which prescribes DD/DTS for surround sound and can easily be transferred to the receiver via passthrough (like when watching a movie). It won't be in high resolution, but at least you have proper surround with no hassle at all. If the surround mix of a particular album is bad you can always switch back to stereo, which may or may not be high-resolution.
If you want to play SACD discs (those have no multichannel DD/DTS fallback) in surround you're going to have to look into a card that supports Dolby Digital Live! and/or DTS:Connect. This will dynamically transcode everything to DD/DTS so it can be properly passed through S/PDIF.
When I still used S/PDIF and discovered HR audio I got myself a nice soundcard with a reasonably good DAC, dumped S/PDIF altogether and went (back, in a sense) to analogue. The audio quality should not suffer and could be improved if you go analogue. As long as you get a card with a DAC that is better than the one in your receiver. With analogue you won't be bound by those weird restrictions that are imposed by S/PDIF. Especially in hindsight, S/PDIF is nothing more than a dirty workound to get digital mutichannel audio crammed though one cable by lack of enough bandwidth.
Edited by LB06 - 6/30/12 at 9:40am