I use desktops for: 8GB of RAM (cheap DDR2!) for running virtual machines, 4-way RAID-0 drives, and dual monitors. This is both at work and at home. At home I also have a gaming graphics card - 8800GTS-512, but I actually play very infrequently (Witcher, Oblivion), so I might even give up on this altogether. I also prefer sound from Soundblaster Live 24 over typical laptop computers.
I actually have 2 similar desktops, one with Linux as a base system, and VMware Server for virtual machines, other with Windows as a base system and also VMware - this one has good graphics card for gaming. The nice thing about having N desktops are "cyclic" backups - I backup one desktop to another, without using external drive.