Hindsight is a wonderful thing
Well your plasmas resolution is simply not capable of exploiting HD and that's basically the end of it, there may be a slight benefit having a HD STB feeding the plasma a downconverted feed but the cost would make that impractical.
I think HD (on Dsat at least) will be playing a major part in UK broadcasting within a couple of years with both PAY and FTA sources on offer not to mention pre-recorded media and huge marketing from the likes of SONY and perhaps long term MS.
HD on DTT will take longer so your 5-8 year projection is on the money and ties in with analogue shutdown.
The only up side is that you have an excellent example of a low res plasma and when you do come to replace it HD hardware will be common place and price wise peanuts compared to today

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