Let's say you have 2 speakers , each of 8 ohms ... If connected in parallel, they'll will give you 4 ohms total , or 16 ohms if connected in series.
I can explain in details if you don't know how to calculate the total impedence , just tell me. :)
I'm not experienced with different types of speakers , but from what I hear , celestion is your best choice ... Most high end amp manufaturers use them .