Any modern motherboard should support S3. The Asus P5K-E/WIFI-AP definitely does though I've read some complaints on not entering or resuming from sleep but on the most part when users post about sleep issues it is device driver issues at fault, not the BIOS. You should still specifically configure S3 over S1 in your BIOS, this is typically found under Power Management ==> ACPI Suspend Type. If you will be using XP, XP does not install the S3 drivers if it is not enabled so try to ensure this is setup prior to installing your OS.
btw, is this a HTPC dedicated system? If so, there are cheaper "better" motherboards available.
Edit: Nevermind, I reread your post and saw that it is a gaming machine as well.
.:: Vista Ultimate x64 :: Antec Fusion :: Corsair HX520w :: ASUS P5E-VM :: Core2 Duo E8400 3.0Ghz :: 4GB RAM :: 128GB SSD System\Apps :: 1TB\5TB Data :: LG GGC-H20L BD/HD DVD :: HDHomeRun x2 ::.