im looking at the dell xps 8500 or 8700, or something equivalent. I like all the stock features, but will eventually upgrade to an amd r9 270x graphics card. do the dell models support crossfire? also, is the motherboard such that I can install 2 graphics cards? idk much about PCIe slots. And finally, are these computers able to fit a new PSU and second grahics card and fans (if necessary) inside the case?