This isn't necessarily true. I've been running
a somewhat
stripped down (no compositing or Nepomuk/Strigi)
KDE4.4/Debian Squeeze installation on a ~6 year old Thinkpad
T42 (PM(a)1.7GhZ) for the past year or so. The system has
'only' 512MB of RAM, and rarely uses it all! (idles at about
200 in fact.) I have another modest system running KDE4.7 on
SUSE that idles at 220MB used. I haven't done any real
optimization on these systems, just disabled desktop effects
and Nepomuk/Strigi. Now, I still wouldn't use it on anything
*really* old, but KDE4's certainly more capable on older
hardware than it gets credit for.
I don't consider any system with a CPU of 1 GHz or faster to be old. :)
You say only 512 MB of RAM. I'm curious what system services are running or what the
system is like when running several apps.
Darrell