There are a number of factors involved. A wiring diagram of your house would be a huge help - # of component, hdmi, coax, and cat5 going to each room, types/resolution of TV's etc. If you have 8 tv's, you might want to consider a few distribution amps. They're normally $150+ but you can find some good deals ($20-60) right now on ebay for Calrad 40-934M units pulled from all the Circuit City stores that closed not too long ago. If you have some tv's that don't have component inputs or don't have wiring run to those rooms, you can always use modulators over coax. If you want to centralize the MD's, you will need to have either hdmi run, use hdmi over cat5 baluns, or use HDFury2 HDMI->Component converters. If you want to use 4 MD's for 8 tv's you will definitely need the distribution amps and the HDFury2's (best price at Monoprice.com).
:Matt