Never buy an Alienware desktop - ever! When it comes to cost and performance, it's always better to build it yourself. The same thing goes for all the other companies, like Falcon, Voodoo, etc. With them, the highest customizations can get to a good twenty thousand USD.

On the other hand, I have no experience with notebook prices and building, but I think it's still better to get an older notebook and upgrade it how you need it.