Well first let me say that I purchased the Dimension 8200 with the Ultrasharp screen, a great laptop. I wanted to get one of the Clevo's, but since my grandfather paid for it for college, he insisted it would be Dell.
If you go with the Toshiba, you should probably not upgrade anything that you could through a third party that would be cheaper. For example, I got an extra 256MB stick of ram for 100$ at crucial; dell wanted 250$.
As for the Alienware laptop, there have been differing opinions. Check out the site
www.clevo.com.tw (i think that is right, but im not positive) for info about the "alienware" laptop. My recommendation is to go to
www.powernotebooks.com to get the exact same laptop for a lot less. People say the alienware version has more venting holes. If you check out clevo, you'll see that all of them have those holes. PowerNotebooks just doesn't have many detailed pictures. The only reason you should go with Alienware is if you live in Nevada or feel the need to be color coordinated for an extra ~150$.
Clevo also has a new 8800 laptop( the alienware is a 5600) that has the second largest screen on the market like gateway's. It even comes with a TV tuner and a flash memory based portable music storage thing that can also be used as a suplemental storage for your PC.
I can't vouch for using PC CPU's in a laptop, but I doubt it will cause a problem. All of the major manufacturers are coming out with them. Dell just got one. I think IBM has one. The only thing to worry about is power drainage, but if you have a high power laptop, the battery life will stink anyway. If I had to do it all over again (and spend my own money), I would have gone with a Sager from powernotebooks or one of those nice slim portege's from toshiba.
Just a thought. If you really want color on your laptop, you could remove the plastic casing and paint it

Start a whole new trend. Case moding for laptops. I can see it now....