hey there long time listener first time poster :P if you looking to upgrade we will need to know what setup your running atm man,

what amp you have (brand and model number would be great )
what subs you have (again brand and model number will suffice)
what wattage you are running through the subs and if you know what impedence your amp is being used at so you/we can figure out if your gonna need to upgrade your amp aswell

also what price bracket your gonna be using
try to remember the more exspensive the sub the more exsepnsive it will be to run like technik said, dont bother buying 3k watt subs if your amp cant even burp a 600 wattage lol

i would say you are best to buy second hand as its easier to get things at a cheaper price and there are plenty of affordable subs and amps that will blow your sox off

look forward to helping you out, Alex