At 12' if you are going 58/60" you probably want 1080p panel...
Pioneer Elite 150FD and 6010 seem to be out of your price range.
Samsung 5884, Panasonic 58PZ750 and 58PZ700 - you need to see them to decide what looks best for you. From my observations in several stores I'd take Samsung 5884.
768p maybe still an option as well, Pioneer 6070 and Panasonic Professional 58PH10UKA (does 1:1 pixel mapping, maybe great for HTPC) are priced very low.
If you are going 50" then it doesn't matter 1080p or 768p as you most likely won't see any difference from 12'. Pioneer Elite 110FD (1080p) is probably the best and 1150 (768p) is probably second best. Closely followed by 5010 and 5080. Take Pioneer out and Samsung 5084 would be my next choice...
Unfortunately today whatever plasma you take if it's not Pioneer it doesn't have the same blacks and this black is what makes any Pioneer to stand out when it's side by side with other brands.
Would I pick Samsung 5884 (size, 1080p, good colors) over Pioneer?
Some time ago (having read all the threads saying BIGGER IS BETTER) I'd say yes until about a week ago I went to Bay Bloor Radio in Toronto and there they had Pioneer Elite 1150 on one side and the wall of Panasonics (including 65", 58PZ750 and 58PZ700 on another side showing the same HD content...
I moved between the panels getting closer and farther trying hard to get the Wow from 65" and 58" Panasonics, but my eyes took me back to 1150.
It's PQ to my eyes was simply better than any Panasonic on that wall.
So now I'd say select the best PQ for your eyes and then take the size you can afford.
But sure, if I wouldn't see any difference between 5080 and 5084 I would go with 5884.