First thing is first:
System requirements are irrelevant. It's the exact same as the x86 version of the operating systems, if not less. All you need to know is whether your CPU supports the x64 instruction set (which is often referred to as AMD64, EMT64, x64-86 and less often INTEL64). And then fresh install the 64-bit OS over.
You'll never go back!
My Computer
Computer type
PC/Desktop
OS
Windows 8 Pro x64
CPU
i7 3820 @ 4.68GHz
Motherboard
ASUS Rampage IV Extreme RoG BF3
Memory
F3-12800CL9D-8GBXL (32GB)
Graphics Card(s)
GTX 480 SLI
Sound Card
Auzentech X-Fi HomeTheater HD
Monitor(s) Displays
Sony 32V5500
Screen Resolution
1920 x 1080
Hard Drives
LSI MR9260-4i (RAID10):
Toshiba DT01ACA300 x 4
iaStorA:
OCZ Vertex Enterprise 120GB
ST3500320AS 500GB
Intel 520 Series 120GB
PSU
OCZ ZX 1250W
Case
HAF X
Cooling
H80
Keyboard
Cyborg V.7
Mouse
Razer Lachesis 3.5G 5600dpi
Internet Speed
23296kbps ds / 812kbps us ADSL2+
Browser
Mozilla/5.0 (Windows NT 6.3; Win64; x64; rv:32.0) Gecko/2010
Might be nice to know the x64 drivers exist for all the hardware *before* the fresh install. Also should download them all so you have them handy if needed.