Might be a good idea to look into building your own. That being said I have an Acer 7720G (gaming, video, sound all optimised) It's almost two years old and it's still keeping up so if you take a look at the G range you can probably get something pretty good. It won't be amazing but alienware just fucks you right in the ear for pretty much the same stuff as you can get from other companies for a quarter of the price.
I'd say buy from a retailer rather than online, it'll cost more but if it craps out you can take it to a store, talk to a guy and see what can be done that day. Online you could be looking at weeks of bullshit.