Well, as for why, because you want it and can afford to buy one hopefully.
Why not? I don't know, based on your post it seems like a waste of power to me and you'll probably be better off with an iBook G4 which according to some...
Performance-wise, definitely go Sager.
Depending on which battery you got the Dell might get more battery life but at that price and judging by your configuration you'll be happier with the Sager.
Won't find anything to match the powerbook at that price, but at a minimum the Inspiron 8600 will do just fine with the current 25% off, it even has a 64 mb nvidia card which is considered best for opengl.
There's always the U1 series.
http://www.vaio.sony.co.jp/Products/PCG-U101/
Has, a keyboard, but probably still too small (and expensive.) Its the only laptop I can possibly imagine fitting into a pair of pants' pockets.
When Dell hits the big coupons and you're not too picky on quality they're fine.
The laptop I have costed me only $750, and Dell gave people a free upgrade on their processor (probably because they ran out of a batch.)
Don't get me...
