Lehet C2D processzoros gépen gyorsabb a 32 bites win10, hiába van 4GB RAM, mint a 64 bites?

Igen, lehetséges.

"Windows 10 32-bit is more likely to work with older hardware and software"

"Windows 10 64-bit includes a few additional security features that aren’t available in Windows 10 32-bit. For example, 64-bit versions of Windows require that all device drivers be signed. Driver signature enforcement means that you can only use drivers that Microsoft recognizes. This prevents malicious or buggy drivers from being used on your system.

Security features like driver signature enforcement are good, but older CPUs may not be fully compatible with Windows 10 64-bit, even if you were running Windows 7 or Windows 8 64-bit previously."

[link]