i know you asked about the dual i7, but here's what the quad i7 in the mini server can do:
handbrake 0.9.5 on high profile preset to encode big_Buck_Bunny_1080p_surround_FrostWire.com.avi (at the 2 minute mark into transcoding:
on the appletv2 preset, handbrake finished the encode in 6:35 (min:sec)
handbrake utilizes all cores and threads, and cpu utilization jumps around between 60%-98%:
handbrake seems to be nice about not hogging every last available cpu cycle if other apps need it. i don't know about the dual i7, but on the quad i7, i can transcode with handbrake and watch a 1080p of big buck bunny playback smoothly simultaneously:
Quad i7 8GB ram geekbench 32-bit: 8911
Dual i7 4GB ram geekbench 32-bit:
6927
quad i7 is about 27% faster (if you factor in ram difference)
Quad i7 8GB ram geekbench 64-bit: 9660
Dual i7 8GB ram geekbench 64-bit:
8257
quad i7 is about 17% faster
so based on these numbers, you can probably extrapolate how much faster a dual i7 is than the core2duo in your macbook pro.