I've had difficulties with power management on my P4B266 mainboard (yes, pretty old but still works).

Here are a couple tweaks I've done (which work with Debian lenny) :

  1. passing acpi=force to the kernel to enable acpi
  2. tweaking the HAL suspend quirks (see bellow) in /usr/share/hal/fdi/information/10freedesktop/20-video-quirk-pm-asus.fdi
  3. tweaking s2ram's options with S2RAM_OPTS="-f -a 3" in /etc/pm/config.d/defaults
Here's the patch on /usr/share/hal/fdi/information/10freedesktop/20-video-quirk-pm-asus.fdi for hal to be used by pm-utils :
--- /usr/share/hal/fdi/information/10freedesktop/20-video-quirk-pm-asus.fdi.orig    2009-03-28 19:29:39.000000000 +0100
+++ /usr/share/hal/fdi/information/10freedesktop/20-video-quirk-pm-asus.fdi    2009-03-28 19:29:44.000000000 +0100
@@ -108,5 +108,12 @@
       <merge key="power_management.quirk.none" type="bool">true</merge>
     </match>
 
+    <match key="system.firmware.version" prefix="ASUS P4B266 ACPI BIOS Revision">
+      <match key="system.firmware.version" contains=" 1010">
+    <merge key="power_management.quirk.s3_bios" type="bool">true</merge>
+    <merge key="power_management.quirk.s3_mode" type="bool">true</merge>
+      </match>
+    </match>
+
   </device>
 </deviceinfo>