I don't think desktops will get smaller in the next five years. Yeah, you'll always have those people who want a Shuttle case, or the business users who can have a little desktop case, but I think real computer enthusiasts will stick with the large towers. It's just hard to work in a small case, simple as that.
I did the same thing before i bought my 8880. I went to the big brand retail stores in the area and looked at their laptops, and I went to all the brands' websites I could think of to configure a laptop online with the same specs. The only thing that came close to the Sager's specs was a Dell Inspiron, and that came out to about $3800, before shipping. No flipping way. Of course Alienware had their system, but it was only up to 2.2ghz, and I wanted a 2.4ghz machine, and I wanted the big boy.
Now I have had it 10 months already and it's still all the computer I need. I still get wows, particularly over the gig of ram, and I just love it when one friend of mine comes over and tells me he wishes he had a laptop like mine. Happens every time, never fails. Now that he is getting some money, I have been showing him pctorque and powernotebooks.
There is a good deal of sense in waiting for the 8890, there really is. Even if you don't get it, the price of the 8887 will surely drop quite a bit because of it. However, it is your money, and if you can't wait, go for it.
I know, I couldn't wait even 2 weeks for the 2.6ghz processor to come out. hehe