wardy22 :
what makes you prefer the HDX? is it the CPU or GPU also, how hard is it to replace a laptop processor?
I just like the build better, and I've owned both Gateway and HP and like them both.
But I just prefer the HDX's overall the look and feel of the 18" model (my favourite was the 20" which died before it got the horsepower it needed to drive the 20" 19x12 LCD).
For raw power the Gateway is better, but it could use some tweaks, like a higher resolution screen.
A little more balanced at a little more money, the MSI;
http://www.msimobile.com/level3_productpage.aspx?cid=6&id=46
But it's been out of stock since appearing on the site essentially.
There is a lower version with the HD3850 for about the same as the Gateway, and has a better LCD, but the graphics solution is a bit lower than the Gateway, and the CPU is lower in a Turion (which also limits upgrade options).
As for upgrading the Processor, it's not as easy as on a desktop, but it's not rocket science and as long as you take it slow and deliberate, it's quite doable.
Main thing is that the options are good enough as a starting point, and then worry about CPU as you go along, if you want more power right away it's usually cheaper to pay for something just under the top like a T9400 or P9500 to begin with. But between a P8600 with GF9800 GPU and T9400 with a GF9600 GPU, I'd take the better GPU since it's the biggest limiting factor in most games and nearly impossible to upgrade (and more complex than CPU when possible).