Xubuntu:

• CPU: 300Mhz+
• RAM: 192mb+

Luit Linux (which uses Xfce) recommends:

• CPU : Intel Pentium I
• RAM : 32 Mb

OSNews.com in reader comment dated 2005

• XFCE - Minimum: P133 with 32mb ram

Wolvix.org

• User recommends at least 192MB for XFCE4. Another recommends +200MHz and 96MB RAM. Note that Wolvix is a livecd and has a ramdisk.

Kwort

• Kwort is a slackware-based operating system featuring the Xfce4 desktop with Kwort's own Network Manager and kpkg package manager. Its minimum requirements are:
  1. Processor: PC i486 or above.
  2. RAM: 16Mb for base system (minimum). 32Mb for desktop (minimum), 64Mb recommended.
  3. Disk Space: 200Mb for base system, 1.5Gb for full desktop (Openoffice 2.0 included)