What's your budget? Just encoding - so CPU intensive? I've just splashed out on a Samsung RF711 for doing encoding to DVD from several formats of holiday videoing, DV tape etc using Sony Vegas. Have been meaning to do this for a while, haven't taken delivery of the machine yet. Samsung have a VAT back offer going on at the mo which made the choice a little easier, combination of cash back on Quidco and VAT back has this coming in at £681 for me (list is closer to £840):-
Intel® Core™ i7-2670QM (4 cores, 8 threads, 6M Cache, 2.2 GHz to 3.10 GHz with turbo boost)
8 GB DDR3
HD+ 17.3" LED backlight 1600x900 display
Blu-ray player/DVD read/write
NVIDIA GeForce GT 540M - 2 GB GDDR3
1TB HDD
You could spec up a non-Blu-ray (I don't particularly need Blu-ray) Dell Inspiron 15 with the same processor, lesser 15.6" screen (1366x768), slightly lower graphics card for £100 or so less, so it'll still smash through the encoding at the same rate just not be as beefy on playback, but it'll still be pretty fine.
The Samsung has a second drive bay too so I'm planning to put an SSD in to make the encoding quicker.