Want 3 monitors - can only configure 2 in XP Pro

Hello,
I have an Asus M3N78-VM Mother Board with 2Gb Ram; AMD 64 x 2 Dual Core 4800+ (2.50 GHz); GTX650-DC-1G5 & 3 monitors (2 DVI + 1 HDMI). I am using XP Pro with SP3. I can select any 2 of the 3 monitors, but cannot select all 3 at same time. Only 2 displays show up in selection Control Panel window. How can I make this work or is it time for an upgrade? No gaming - this is a work station for business only. All help is appreciated.