The A51m is just Alienware's version of the Clevo 56N. There are many other sites offering the same basic system, just tweaked differently. The system in question is most commonly known as the Sager 5620 (or the P4
m version - 5650). You can find this system at the following (and other) sites:
powernotebooks.com
xtremenotebooks.com
FOSA.com
sagernotebooks.com
(ProStar's site - I don't know the address, and it will be called the ProStar 5694, or something like that, but Sager and ProStar are sister companies, anyway)
Alienware is the only one of these who offer custom paint jobs, though.
They're also by far the most expensive...