Almost always, it'll settle around 8000-8200Kbps.
And sorry.. I missed that in your first post..
I think the problem is in the plugins and the real capabilities of the Sling Adapter.
The sling adapter comes with a manual all of 1 sheet, with a supposed speed declaration of HD requiring 3 meg or more for bandwidth. Obviously as you've seen, you're pushing 8 meg or more.. my screen cap showing 8150 (8.1 meg) and I've had streams as high as 10 meg. Yet as soon as I set it for Best/HD ... while the picture improves, the audio slightly improves... within 10 seconds I start getting audio sync problems, then choppy video and it never gets above 11 meg on mine ...
I've proven I can push 18 to 22 megs of traffic across my wireless bridge (streaming on "Better" @8 meg, and moving a 500 meg iso at 10 to 14 megs) so this speed limitation isn't my computer, my network, etc.. but that something with the video data path.. be it the Sling Adapter isn't able to process, or a subset of features that come on higher end systems (quad core rather than dual, 4 gigs of ram instead of 1) or just the software itself is the reason *i* can't do HD in house.
Since your machine sounded more advanced .. its more than likely the plugin just won't activate HD properly, or what ever it needs to do, to get the stream to you... I would expect the stream to hit 16 to 18 megs for a full MPG4 HD stream (16 meg is quoted from other sources) and either the adapter really can't do it, or there's some combination of things that is not allowing the mode to be switched into, courtesy of dish/sling and its "administrivia" controls over our connection.
Would your video improve with a different video card and driver? sure... they have different feature sets different anti-alias and other features that are there to make the video look crisp and off load processing to the card.. but would it be worth it? would it really make the difference to getting the sling adapter to do HD mode? doubtful..
I'm laying odds that initially the sling is adapting to what its getting told is workable ... and until they give real meanings to the controls... there won't be a way to get it into true hd mode reliably.