Having originally started this topic, the results of my trip can be found at
http://www.birdforum.net/showthread.php?t=57061
From talking to people and getting lots of helpful info on this thread (thanks everyone!), Carn Ban Mor seems like it would be a good bet for Ptarmigan with a well defined area at the top to look in. The scenery seems pretty good and you would have a good chance for Dotterel. However, it is a pretty long walk and constantly sloggging uphill. Still, if you have most of a day, it might be well worth it.
Given the time that I had, I opted for Cairngorm. If you take the path to the right of the ski lifts you can, without too much hassle get to Coire an Lochain, where we had excellent views of Ptarmigan. The scenery is still pretty good but from the accounts on this thread, not as good as Carn Ban Mor. It didn't take us long to do the walk (left at 12 and were heading back down the road by 3) and it wasn't at all strenuous (although that I suppose is a matter of opinion).
For Crested Tit, we did the track to Loch Mallachie but actually had one by the Osprey Centre Car Park. I'd agree that Loch an Eilean is a good spot as well, having seen them there on a previous trip.
Capercaille was easy - due to the dawn watch put on by the RSPB. I'd agree with Capercaille71 that you'll be very lucky to get one at that time of year though.
We had Scottish Crossbill in Grantown-on-Spey woods. I'd recommend trying Abernethy, Loch an Eilean and Grantown-on-Spey and hope to get lucky.
Good luck and enjoy your week (oh the luxury of a whole week!)
Matthew