one of the biggest limitation is harddrive speed. a spinning hd has its limits however here is the future
http://atomchip.com/_wsn/page4.html
solid state harddrive
1 terabyte of ram
and 2 terybytes of space (harrddrive)
all running at motherboard and proc speeds.
i have to get me one of those and a desktop like that.
imagine what it could do as a server. no harddrive getting busy because of hits
it can probably do like 97870870 gigs LOL