I do a lot of pc gaming, but that's not what the upgrade would be intended for. Lately I have been working with 1080p HD video. I love the Core 2 Duo, but I think the quad core would really boost the rendering speed.
Went from an Intel Dual Core to a AMD Quad and the difference was night and day for rendering 1080p. Of course the video card has some responsibility but on this particular system I'm running an older Nvidia 7600GT and it works great. My older proc was definitely bottle-necking my system for 1080s.