Any computer that is at least a P4 (any speed) makes an OK cow.  A faster P4 with 1 to 2GB memory makes a great cow!  Memory helps, but as long as it's a cow only, you only need enough memory to meet the minimum specs for the operating system.  You need a graphic card, but nothing fancy.  Also add into the price the network setup.  You need a router and a network card for the cow.  If you don't want /need a monitor for the cow, then get a KVM switch.  You need to be able to boot up the cow and be able to install software on it, so that means you need a monitor and keyboard.  A KVM switch just allows you to switch between your main computer's monitor and keyboard and the cow. 

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