According to the Asus web site, the K8N4-E motherboard has 4x SATA 3 Gb/s ports.
It supports RAID 0, 1, 0+1, JBOD.http://www.asus.com/Motherboards/AMD_Socket_754/K8N4E/#specifications
I can't answer your Win XP question, though.
BTW, WD drives have an OPT1 jumper that restricts their SATA interface speed. I don't know how it works on 6Gbps drives, or if it works at all.
Jumper Settings for WD SATA and EIDE Hard Drives:http://www.wdc.com/wdproducts/library/other/2579-001037.pdf