X5260 = 80W TDP
X5450 = 120W TDP
X5450 = 2x120W TDP
So that would be the order heat/noise.
Converting a HD MPEG2 to H264 on the 840 takes ~143s, the X5260 ~80s and the X5450 ~50s depending on if the encoder can use 8 cores then the dual X5450 could be faster. This is based on data from tomshardware cpu charts Q1/08 for desktop CPU's.