I also worried about how big on the CPU. I went i5 and am happy to have done so. Since I started doing some transcoding, I see some programs that task even my i5.
Doing stuff like going from BDMV to mp4 or in .mkv using DVDFAB for blu-ray will put my CPU usage in the 90% range. Some programs seem to not be written as good like Pavtube and will always task your cpu. As SsZERO stated, I don't see a blip in cpu usage when decoding HD video to play.
If you still have the chance, I would opt for a dual core i5 vs i3. I don't know much, but I have also read you should not go to a quad core as each individual core is smaller and will not help transcoding. If you do not plan on any transcoding, then even i3 may be overkill. Just one opinion, maybe good or bad. I would listen to SsZERO
W7(64) HTPC 16g ddr3 ram, i5 2500k cpu, GA-Z68XP-UD4 MoBo, Gigabyte 6770 gpu 1g ddr5, 2 Hauppauge 1250 tuners 6TB Storage, nMediaPC 5000B case, 2 Dual Silicon Dust HD HomeRun network tuners
WHS 2011, 8g ddr3 ram, i5 650 cpu, GA-H55M-UDH2 MoBo, Sapphire 5670 gpu 1g ddr5, 16TB Storage (My Movies Server)
WHS 2011 8g ddr3 ram, i3 2100, Asus P8P67-MPRO MoBo, 2 Hauppauge 1250 tuners, 4TB Storage(Sagetv Server)
(2) Hauppuage 1212 HD PVRs hooked to HTPC
2 (SageTV HD300)