Your system should be able to run Windows 7, although it may not seem very snappy. I am also still running my Socket 939 rig with Windows 7 64-bit. My system is in my sig.
------------------------------Athlon 64 X2 4400+ @2.4GHZ,Zalman CNP9500 LED CPU cooler Asus A8N32-SLI Deluxe, 2GB Corsair XMS-3500LL w/ activity lights, Nvidia 9800GTX 512MB, 2 x 400Gb Western Digital Sata drives, Audigy 2 Zs, PC Power and Cooling Turbo Cool 1Kw
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I have both XP and Windows 7 on the same motherboard and everything's fine.
In a lot of ways there's nothing wrong with XP so be sure of your reasons for upgrading. XP is no longer supported which means certain security upgrades and other things.
I would definitely get the OEM version of Windows 7 x64. You could also dual boot but I haven't been back to XP since switching.
You may wish to add 1gb or 2GB if possible. Make sure it's the same type and latencies (7-7-7-24 or whatever you have).
Your processor is an Athlon 64, the 64 means 64-bit, so yes you can use the 64-bit version of windows.
------------------------------Athlon 64 X2 4400+ @2.4GHZ,Zalman CNP9500 LED CPU cooler Asus A8N32-SLI Deluxe, 2GB Corsair XMS-3500LL w/ activity lights, Nvidia 9800GTX 512MB, 2 x 400Gb Western Digital Sata drives, Audigy 2 Zs, PC Power and Cooling Turbo Cool 1Kw
Reply to HVdynamo