Most HD movies arent this big, but just over 2 hours of 1080p is 20-25Gb.
24Gb = 4 hours on my connection.
End of the day most people wont be downloading HD stuff unless theyve got a big enough screen. 720p is good enough upto 32". 37" (although more like 50+) and youll start to notice the difference with 1080p.
Roll on 52" 1080p screens this year for £2k