I have the 170-500 and Sony A100.
It's virtually impossible to tell from an 800x600 image what is causing your woes.
If I had to make a guess I'd say camera shake. Even on a tripod with a 750mm (eq) lens at 1/320s you have the potential for movement. Did you turn SSS off because it was on a tripod? Leave it on, I've been amazed at how easy it is to move the camera on a tripod.
Have a close look at the full size image. Can you see a sort of ghost image of the perimeter of the main objects? If so it's going to be camera shake.
Wide open at that range you will have very little depth of field and the A100 focus accuracy isn't all it could be. Have a look at the fence rail at full size and see if there is a point in front or behind the bird that is in focus. That would suggest that limited DOF and focus accuracy are a problem.
Much is said about the noise handling of the A100, some of it justified and some of it just hooey. It's fair to say that you will need to invest in some noise reduction software, I use Neat Image. But it's also fair to say that at iso 400 and 800 on a well exposed image the A100 records more detail than any of it's contemporaries that I've ever seen.
Sony have chosen to adopt conservative settings for noise reduction and sharpness. It records a lot of detail and stops the image looking plastic. But you have to know how to bring it out. When I first got mine I was apalled by the high iso quality but having got the hang of it now I really like the detail.
So here's a few tips I've learned in the last 2 years of A100 ownership:
Leave SSS on all the time, if it isn't needed it won't be used.
Use DMF focus mode when shooting wide open, holding the button autofocusses and then disengages the AF so you can fine tune with the manual focus ring.
Use the focus area select, exposure follows the focus point so if you lock focus and recompose the shot will be exposed for wherever the focus point is now, not your subject.
Don't be affraid to use iso800 if the light is bad, be carefull not to underexpose and iso800 shots can be excellent.
Use NR software to clean up the image (I find all Sigma lenses produce noisey bokeh?) and apply a bit of subtle sharpening.
I'm crap at wildlife photography, it's really not my thing but even I have managed some good sharp shots with this combo.
The first couple of shots here are with the beercan but all the rest are hand held with the Sigma.
http://public.fotki.com/spannerdude/wildlife/