Welcome, first your sort of going about this backwards, yes the center is very important and about the hardest working speaker during movies but you need to pick the front L&R first and that dictates the ideal (though not written in stone) matched center. So decide if you want smaller stand mounted fronts or a floor standing speaker and try to listen to anything before you buy. Looks are important but you can eaily get a great looking speaker you cant stand to listen to so hearing is the best advice I can give.
Once you decide on mains you can explore a matched center or use the reciever in phamtom or stereo mode, add a matched or any center to get you by till you can save and ofcourse the surrounds can usually be matched to fronts even later but surrounds are not nearly as important to match so dont get to caught up in that right away, many dont match center for any number of reasons from budget, looks, size or personal choice and get excellent results so take note of that option aswell. If you are going to go the small stand mounted (monitor) route it would be ideal to get a speaker that atleast will give you 80HZ of bass,,,,,,,,,THX standard performance. One more tip is pay attention to how efficient speakers are, over 90db is ideal for a reciever to power with ease and you can always ask around if you list specs of any possible purchase to folks here for more advice.
Good luck and email if you want any help........cheers