It depends on whether you intend to overclock. What size case and motherboard? What do you intend to use the computer for? How many cards? Is it to be a production machine? A game machine? If you intend to install a new MoBo and CPU in your old case, it depends on what it is.
Mine was easy. I overbuilt it, because I wanted longevity. It is quite vanilla. I'm still on Chimera/ [url="http://www.insanelymac.com/forum/topic/279450-why-insanelymac-does-not-support-tonymacx86/"]#####[/url] for the moment. I'll try a Chameleon install when 10.8.5 arrives. I can experiment, because I have three other HDD's and a SDD.
Intel H4000 internal graphics for now
16 Gb memory
Corsair Carbide 500R case
Corsair enthusiast 650Watt 80+ Bronze PS
Seagate 1 TB 7200 rpm HDD
Sandisk 120 Gb SSD
I've read of Chameleon and Clover installs for it.
But you can build a nice and simpler system for around $800. A i5-3570K and a GA-Z77-DS3H will run you a bit over $300.