Why? I'm running it on an old machine, and it is fine.
The newer FireFox that's coming out is even trying to reduce it's memory bloat. I seriously have the impression that if a machine ran Linux OK in 2001, that so long as you bought RAM upgrade when it was cheap (if you didn't get some 2nd hand from dealers recycling biz pcs), that the system will be equally quick today, running the latest and greatest Desktop aimed at Businesses.
The fact is, the modern CPUs are waiting on memory access and disk most of the time, rather than "processing". Most Biz PCs tend to use integrated graphics, nothing special CPU, standard RAM, and not be the over-specified Gaming Monsters that enthusiasts (and PC Mags) tend to specify.
The $200 Walmart PC is Via C7 1.3Hz based and got pretty good customer feedback, gOS is based on Ubuntu, they just moved to Enlightenment rather than GNOME WM to be more lightweight.
http://www.google.co.uk/search?q=gOS...UTF-8&oe=UTF-8
The Mobo + CPU developer kit is about for $60 + $18 for 1GB RAM, if you're not Windows encumbered, and can use a screw driver, it's hard to have a really slow PC these days.