I just upgraded myself a couple of months ago from a Q6600, Gigabyte P35 DS3R, to an i5 3570K, Gigabyte Z77X D3H, plus new cooler (in sig) for $360. Our graphics card are probably closely matched (6870), I couldn't be more impressed with both CPU and mobo, and my graphics card has had new life breathed into it too, getting a very nice bump in frame rates in every game, at least 20+. I honestly didn't think that it was my Q6600 bottlenecking my system, but it surely was.
Are you planning to go Intel or AMD? Either way I don't think you can go wrong with ASUS, MSI or Gigabyte motherboards these days, and if you get a CPU for around the $200 mark, that leaves you a very broad selection of sub $200 boards to choose from which makes it a lot harder to narrow one down. Even a sub $150 motherboard might be a good idea as you will need an aftermarket cooler too, the stock heatsinks aren't really worth the TIM that comes on them, so if you leave yourself $50 for that you should be golden. Then you might want to add another G/card in the future so an extra GPU slot is a big plus for future proofing.
Check Tomshardware CPU charts, they give a good indication of CPU ability for the things you use or might use them for. Multitasking is probably more suited to an AMD processor where games and single apps generally favor Intel, the difference is probably negligible though so its just personal choice really.
CPU bench charts. http://www.tomshardware.com/charts/cpu-charts-2012/benchmarks,140.html
As a rule of thumb I personally always stay away from the latest tech, I figure a good year or two of other people testing the new stuff reveals the price/performance sweet spot for the rest of us. Granted you are not wanting to overclock straight away but as you know there will come a time when you will be forced too, that's why I went for the i5 3570K, its got the power right now and plenty under the hood for later.