At 9' away and considering you're coming from a 50", I would at least go with a 60", but 55" is OK.
Considering your budget, I would go with a Panasonic 60/65ST50, 60/65GT50 or Samsung PN60E7000/8000. If you want the best and don't mind spending the money, the 65VT50 is the best PDP overall, but as BillP said, price/performance wise, the ST and GT have much better value and offer PQ fairly close to the VT50.
The advantages of the GT50 over the ST50 are: THX cinema and brightroom (decent picture presets), better styling, dual-core processor (apps, menus will be snappier), extra shades of gradation (doesn't really make a difference), and an extra HDMI and USB port. The VT50 has (arguably) even better styling, more picture controls (ISFccc, so slightly better color after calibration) and 96Hz mode (better motion for film/24p). There is some minor flickering with 96Hz, so you might end up preferring 60Hz anyway.
As mentioned, THX modes are decent presets/picture modes that are better than the presets on the ST50. But both the ST50 and GT50 have more or less the same picture quality after calibration. Because the GT50/VT50 have the THX modes, it gives you better options if you want separate settings for day and night viewing. The ST50 can also have two good modes for day/night (using Cinema and Custom modes), but Cinema mode on the ST50 isn't quite as good as the other picture modes (gamma is slightly low).
So basically if you want the best and are willing to pay a premium for slightly better PQ and some extra features, the VT50 is the best Plasma. Between the ST/GT, If you're not getting it calibrated, the features of the GT50 appeal to you and/or you want 2 good modes for day/night, the GT50 is the better choice. If you're getting it calibrated, you only need one mode (or you can live with Cinema mode for night viewing) and the features of the GT50 aren't needed, the ST50 is the better choice. The money saved from the ST50 can be put towards a pro calibration which will have much better picture than an uncalibrated GT50 (or even VT50 for that matter).
I know the Samsungs have slightly better color than the ST/GT (VT50 is better than any of them), but the blacks aren't quite as good as the Panasonics. I would personally lean towards the ST/GT/VT, but the E7000/E8000 aren't bad choices either.