For gaming, the difference should not be too great of a difference, unless the game itself requires a quad core CPU.
As for transcoding, I guess it depends what codec you intend on using. If you are encoding video using the x.264 codec, then a quad core CPU will definitely outperform a dual core CPU. However, if you use XviD, then there won't be much of a difference since the XviD codec basically uses just one core.
I encode some videos for my Cowon S9 media player using the XviD codec on my Lenovo IdeaPad with a Core i5-2410m (dual core + Hyper Threading). Looking at task manager, at best the codec only uses 40% at most and the least would be around 30% (it fluctuates from moment to moment). Encoding video using XViD codec on my quad core Q9450 rig, the load placed on my CPU according to task manager is at most 33%.
Encoding with x.264 on the other hand maxes out my Q9450 and it should also max out my Core i5-2410m since x.264 can take advantage of Hyper Threading. I encoded something last week on my laptop using x.264 (1st time doing so), and I'm pretty sure the Core i5 was maxed out at near 100%.