I know that KDE 4.7.2 and Gnome 3.2 were sluggish on the 2001 Compaq Presario with 512 MB of DDR, 128 MB video RAM, Athlon XP 2100, 40 GB 5200 RPM HDD.
LDXE 0.5.8 is only using 180 MB on the 3.1 kernel at beginning of LXDE desktop shell and I opened all the gnome, kde, and XFCE programs thru it. The resolution is 1280x1024. LibreOffice 3.4.2 only uses 38 MB of memory. Firefox 7.0.1 uses 140 MB, VLC uses 30 MB. With all those open, I was using 400 MB RAM
openSUSE 12.1 with XLDE selected is the best option. There is a LXDE BSD distro. "PC-BSD LXDE" would beat openSUSE in stability and security (newer UNIXs can kick the snot out of Linux). The PC BSD KDE distro uses 1 GB RAM at start up; therefore, "PC-BSD LXDE" would use almost twice Linux kernel memory.
To sum it up, Windows XP SP2 at 1280x768 with 32-bit color has slow periods on a 10 year old PC. EXT4 is more speedy than NTFS. LXDE is more speedy at 1280x1024 16-bit color on 2001 PC. If I download the nVidia driver, LXDE can be faster still.