Depends on the size of your TV and how far you sit from it. I sit between 6 and 7ft from my 37" and although HD is certainly sharper, it hardly gets me salivating with excitement,
Found this which might help as a guide:
The 2-5 principle
Standard
For an optimum viewing distance with Standard Definition material, the nearest seating position should be limited to around twice the screen width while the furthest seating position should be no more than around five times the width of your screen.
HD
For an optimum viewing distance with High Definition material, the nearest seating position should be adjusted to around 1.5 times the screen width while the furthest seating position should be no more than around four times the width of your screen.