Short Answer: You don't need a 64-bit OS to use the entire RAID array unless you want to stay with Windows XP. You'd probably be better off upgrading to Windows 7, be it 32 or 64 bit.
What you really need to do is read the Windows and GPT FAQ. Windows XP x86 only supports MBR-style partitioned drives, and MBR-style partitioning is limited to 2 TB. No matter what you do, you're not going to be able to see any capacity beyond 2 TB for a single storage container.
To use the entire capacity of that RAID array at once, you'd need to use GPT-style partitioning. Of the "modern" Windows OS family (XP, 2003, and newer), XP x86 and Server 2003 x86 RTM cannot use GPT disks at all. XP x64, Server 2003 RTM x64, Server 2003 SP1 (x86 & x64), and all editions of Vista, Server 2008, 7, and 2008 R2 can use GPT disks for data discs.
To boot from a GPT disc, you need an EFI-based system and Vista/7/2008/2008 R2. Server 2003 for Itanium can also boot GPT discs, but that's pretty uncommon. :-)
Theoretically, it's possible do install a driver into Windows XP to allow it to see all the space on your array, but I'm not aware of any that are available. I think Western Digital provides one for their 3 TB external drive (along with a controller), but I doubt it will work with all drives.
Another alternative is to see if your RAID controller will allow you to carve that array up into multiple logical drives and present those to the OS instead of a single 8 TB container. It'd be similar in concept to partitioning the array before presenting it to the OS, so XP could see 4 x 2 TB drives.