theplanet.com will rent you soemthing like that for USD$1,600 per month, plus bandwidth.

You can also make it yourself and colo it. penguincomputing.com has a line called the Altus 600. It's a 1U machine with two CPU slots and 16 RAM slots. Source the RAM from Crucial or PNY, and you can get a 2 core, 16GB machine for about $3,300 bucks.

http://www.penguincomputing.com/index.php?option=com_content...

What do you mean by "clusters"? High-end, low-latency compute clusters (using e.g., Myrinet) or just a rack of machines? Are you doing lots of calculation or lots of data or both? If you have lots of data, what's your storage model? If you're going to have lots of machines, what's your needs for connectivity (NICs, intra-cluster, 'net drops, etc.)?