If you don't need good single thread performance and don't care about Performance/Watt that much you can get used servers for very little money with lot's of DD

If you don't need good single thread performance and don't care about Performance/Watt that much you can get used servers for very little money with lot's of DDR3 memory (128GB+) and these X5670 als dual cpu systems. If you don't do anything fancy you get 80% of performance for 20% of the price of a new system.