I would like to know WHY you are not OCing? It seems to me like there are no valid reasons not to overclock CPUs these days. I'd just go with any 100$ z68 motherboard that has all the features you need and get a 2500k. The overclocked 2500k will perform much better than the 3550. With the stock cooler, and stock voltages (no possibility of damaging your CPU whatsoever). I'd recommend getting an Asrock motherboard.
i reckon it would probably be better to get a 3570k, encase in the future if u want to overclock or anyway it gives abit of a boost compared to a 3550. the motherboard i would go with would probably be the z77x-ud3h. got great features, great price, never had a problem with gigabyte been using since 2003+.
if you really wanted to go with the 3550 i would recommend that over the 3450 and i would go with the z77x-ud3h
Looks like a very good mobo for the price although don't get it if you ever plan on crossfire or SLI. 1 @x4 means it will only run on 1/4 of the bandwidth. Also what are you going to use for hard drive? SSD or HDD? To be honest ssd is pretty damn awesome. I have one in my new machine and map changes are instantaneous. Won't give you more FPS or anything but faster map loading is awesome.