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@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