Rackspace's dedicated hosting is not a great comparison, they're definitely at the high end of pricing.

That said, don't forget that both the Rackspace and The Planet boxes come with 2 terabytes of transfer which would be $300 from Amazon. When you factor that in, suddenly Rackspace becomes competitive and The Planet vastly cheaper.

Softlayer will sell you a quadcore box with 8GB of ram and 4 terabytes of transfer a month for $200.