Do i need the same graphics cards for a crossfire

I am wanting to go with dual PCI-E cards with ATI CrossFire Xpress 3200 I have one card that is crossfire capable and i know i need another, but can i get a bigger cards and give that one more load than the one i have now. Because i cant afford two big video cards right now, And does any one know of some sites that have great deals on hardware. thanks mattm