A lot of encoding, video/audio editing, rendering, CAD, and 3D programs use 4 and more cores. Realistically the only things that use more than 4 cores require true CPU horsepower from Intel and those people can afford such CPUs. For instance Xeon workstations, or even the SB-E Hex chips like mine (hell no, I didn't pay for it lol).
Games that actually use more than 2 cores is very slim and I can't honestly think of any off the top of my head. By use more than 2 cores, I mean 100%, not just some blips across each core. It's sort of like how everybody wants more than 8GB of RAM simply because it's cheap. Sure, but it's a waste unless you can truly utilize it. We are in a period where we have a problem between 32bit and 64bit, and currently due to the majority having 32bit consoles and mostly 32bit OS in your average PC games are coded with mainly 32bit in mind. What that equates to the average PC gamer is, more than 8GB is wasted due to the fact that a game can see a maximum of 3GB with help unless it's 64bit native. Even then, I have never seen a game use more than 3GB of RAM.
As for your other question, I used to have a 6000+ chip in my server. When my friend who didn't have a PC came over I used to pop my old 465 in it so he could play BF3 and it actually ran medium to high settings at 1080p.
Your best option would to be this.
Newegg.com - AMD Phenom II X4 955 Black Edition Deneb 3.2GHz Socket AM3 125W Quad-Core Processor HDZ955FBGMBOX
It's under 100, and will honestly handle any game of today. Your 260 will be the bottleneck. Currently my fiance has my 260 in her machine because I swiped her 5850. It is paired with an i5 750 stock and it handles her games.