First off, what is the model of Hitachi you are looking at.
Secondly, why are you dead-set on plasmas? Perhaps I'm mistaken, but that is the impression I am getting. In terms of the plasma vs. LCD debate, much will be happening within the next few months in the LCD camp with significant improvements in black levels. Also, another consideration is the fact that if you don't absolutely need a set at this point in time, prices will inevitably drop in three months' time and there may also be no-interest financing. This may ultimately trump what you are saving in terms of the 5 years same as cash on the $3500 set you are currently looking at.
As an example, just in the first month alone, the prices on the new panny 1080p sets have dropped substantially... nearly $1000 for the 50pz700u alone. Just my 2 cents. Good luck with your future purchase.