By the time you've replaced the CPU, video card, and memory in a desktop, you've bought yourself a new laptop.

I have desktops, but I rarely use them. I have a MacBookPro which can drive the 30" 2560x1920 apple display (at the same time as it's own). It is huge, more luggable than portable, but I'm out on contract and won't take a desktop. It stays in my room, but has all the power I need.

My Averatec will fit into that envelope they show for the Apple Air, but is thicker. 1024x768, can burn DVDs, and has all the other peripherals (firewire, PCMCIA, ethernet). This is my main machine for development or anything I can't or don't want to do on the n810. It also runs XP for those rare periods I need to do so (config program for hardware which only runs on XP...).

If I needed higher-end computing, 200Gb or more, 4Gb main memory (in my PPC mac!), or N-way CPUs where N is a large power of 2, I'd do a "desktop", but it would really be a server.