Currently I am building my own gaming rig, and I was wondering if I should go for Windows XP Pro or spend $2 more and get Windows XP Pro x64? Is one better than the other for gaming? I don't know if it matters much, but my specs are as follows:

Case : NZXT NEMESIS ELITE BLK Black 1.0 mm ALUMINUM ATX Mid Tower 400 WATT PS2 ATX 12V Power Supply http://www.newegg.com/product/prod [...] 6811146007

Motherboard : ASUS A8N-SLI Premium Socket 939 NVIDIA nForce4 SLI ATX AMD Motherboard
http://www.newegg.com/product/prod [...] 6813131540

Video Card : ASUS EAX1900XT/2DHTV/512 Radeon X1900XT 512MB 256-bit GDDR3 PCI Express x16
http://www.newegg.com/product/prod [...] 6814121554

Power Supply : Antec TRUEPOWERII TPII-550 ATX12V 550W Power Supply
http://www.newegg.com/product/prod [...] 6817103931

Processor : AMD Athlon 64 X2 4200+ Manchester 2000MHz HT 2 x 512KB L2 Cache Socket 939
http://www.newegg.com/product/prod [...] 6819103547

Memory : CORSAIR XMS 2GB (2 x 1GB) 184-Pin DDR SDRAM Unbuffered DDR 400 (PC 3200) Dual Channel Kit
http://www.newegg.com/product/prod [...] 6820145575

Removable Storage : Sabrent 52-in-1 USB 2.0 Internal Flash Memory Reader/Writer
http://www.newegg.com/product/prod [...] 6820300608

3.5 Floppy Drive : SONY Black 1.44MB 3.5" Internal Floppy Drive Model MPF920 Black - OEM
http://www.newegg.com/product/prod [...] 6821103116

Hard Drives : 2 Seagate Barracuda 7200.9 3Gb/s ST3160812AS 160GB 7200 RPM 8MB Cache SATA 3.0Gb/s - OEM
http://www.newegg.com/product/prod [...] 6822148103

Keyboard : Microsoft Comfort Curve 2000 B2L-00047 Black 104 Normal Keys 9 Function Keys USB Ergonomic Keyboard
http://www.newegg.com/product/prod [...] 6823109156

Monitor : SAMSUNG 740N-Black 17" 8ms LCD Monitor 300 cd/m2 600:1 0.264mm Pixel Pitch - Retail
http://www.newegg.com/product/prod [...] 6824001072

CD-RW/DVD-RW : 2 Lite-on SHW-160P6S04 16x8x16xDVD+RW / 16x6x16xDVD-RW / 48x24x48x CD-RW
http://www.newegg.com/product/prod [...] 6827106013

CPU Cooler : ZALMAN CNPS7000B-Cu LED 2 Ball Blue LED Light Cooling Fan - Retail
http://www.newegg.com/product/prod [...] 6835118119

Surge Protector : Opti-UPS ES1000C 8 Outlet 700 Watt
http://www.newegg.com/product/prod [...] 6842107113

Could someone please help me?

If all your hardware has 64-bit drivers to support it, then go for it. That's the only snag with XP-64 right now... there's somewhat of a lack of driver support. Lots of companies have drivers, but there's a few (Logitech webcams) that don't.

Basically go through all the hardware you will be using and make sure there are 64-bit drivers available.