I am planning to buy headphones soon and i am looking at these in the price range I want. http://www.amazon.com/Beyerdynamic-770-PRO-250-ohms/dp/B0006NL5SM I would like the 250 ohm version but I do not know if my speakers will power it. (I wish i could link these but they are ancient). Will average speakers play 250 ohms at moderately loud volume.
You're definitely going to need an amp for those if you want them to sound decent. While they may be a little too much for onboard audio, they're not exactly hard to drive, so you can get away with using pretty much any amp, though I recommend that you go with something decent like a Schiit Magni + Modi for desktop use or a Fiio E12 if you want something portable.
Someone correct me if I'm wrong but the standard impedance level for headphones is 32 Ohms. That is most of the headphones on the market are built so they can be driven by a lower power source (MP3 players, phones, and onboard audio for the most part). I believe that headphones with an impedance level of 80 Ohms can be adequately driven by lower power sources as well but you are going to need an amp for anything above that.
Get an amp for sure.
Get the 80 ohm model if you don't want to buy an amp.