RAM these days.
Just did a reboot of my FC2 workstation, started a terminal emulator and a browser.
$ head -2 /proc/meminfo
MemTotal: 1035692 kB
MemFree: 686080 kB
$ echo $(( (1035692-686080)/1024 ))
341
Including buffers, the system takes 341 MB of RAM just to get going. And you can still buy desktops with 256 MB!
I must admit that Linux 2.6.x tends to be optimized more for lots of RAM, as compaired to 2.2.x or 2.4.x.
Actually, if I was going to recomend a desktop for those who take no pleasure in building thier own system, I'd say go with something like the HPQ SR1000Z. Take the slowest Athlon XP offered and 1GB of RAM. I think this is what I reccomended to my father a year ago, and I think he's happy with that computer.