But here's your serious advice:
I wouldn't worry about the center right now. The mains are more important, and you can always run a "phantom center" if your AVR has that capability in the meantime.
On the new market, head over to newegg and look at the Polk Monitor 60s and 70s. They're one of the best bargains on the new market right now.
If you're looking used, Polk still offers a ton of bang for the buck. Your ears may differ from mine, but i find most of the Klipsch, Infinity, and Cerwin Vega in particular to be good at one thing within your budget: PLAY REALLY LOUD. And that's about it. They're harsh, boomy, muddy down low, etc etc etc. I'm not saying that they don't make good speakers, because they do, but you'll be hard pressed to find their good speakers on your budget.
You may be able to find a pair of Klipsch KG5.5 or KG3.5, and they're good speakers.
Vintage Polks are a great bang for buck. Monitor 7s or Monitor 10s will fit the bill.
Other brands to look at: DCM (someone already mentioned them, i think. Only the TimeFrame or TimeWindow speakers are worth looking at. The later more traditional-looking speakers are worthless other than being in a garage.
Mirage. Canadian company, so you may not see much of it around, but they're a great speaker, and most people in the US don't know about them, so they sell them for cheap.
Paradigm. Yes, you CAN find Paradigms on the cheap. The older Monitor 7se MKiis, 9se MKiis, etc etc etc. You could easily find a pair of Monitor 7s for $150 or less if you're patient, and the entry level Paradigm centers are cheap as well.
Paradigm would be my vote.
Out of curiousity, what AVR/Reciever do you have? Nothing makes mediocre speakers sound great like a decent preamp/amp. AVRs typically have pretty E36 M3ty amp stages.