The GX620 should support Windows 7 64 Bit but the maximum RAM is 4 GB so theres little advantage. Most GX620s don't support Windows 8.1 or 10 64 Bit due to elevated processor requirements. Look for a second hand OptiPlex 760 or 780 with a Dell Windows 7 professional OEM license, it may be cheaper or comparable to the cost of buying a new license for that model and they perform significantly better.
is the Dell OptiPlex GX620 desktop a 32 bit or a 64 bit computer? I'm going from XP to Windows 7 and need to know I guess.
32 or 64 bit