I was wondering if we take just 256 bits of plain text and simply XOR every bit with a 256 bit key, how strong can it be ?
That construction is called a one-time pad or OTP (not to be confused with a one-time password). OTP's are theoretically perfectly secure as long as the key is not repeated or reused and the key is indistinguishable from random by any adversary. In that case any 256 bit plaintext message is equally likely, so it is impossible to even brute force the key. Because of that, the size of the key and plaintext do not even matter, as long as there are as many bits for the key as there are bits in the plaintext.
Note that an OTP does not provide message integrity nor authenticity, only confidentiality. Of course the exact size of the input is leaked by an OTP too, so that's not kept secure either.
Of course, generally the practical security depends on the system, the quality of the randomness, the actual access to the key etc. etc. etc.
As long as the key is perfectly random and you only use it once, the scheme is perfectly secure.
The problem is that whole "you only use it once" bit. If you use the key more than once then the attacker can xor the ciphertexts to obtain the xor of the plaintexts. Since most messages are far from random, statistical analysis techniques can often reveal a lot of information given an XOR of plaintexts.
This is perfectly secure (one time pad) as long as the key remains secret and is used only once but not very practical. The problem is that instead of a safe channel for transmissing the message you will now need a safe channel for transmitting the key, or be limited in the total length of messages a recipient can decode before both sender and recipient run out of key.
In practice, many public key encryption protocols actually work with a one time key used for one transmission (the "session key") that is generated for the sake of the session and transmitted using the public key encryption method. So one (or rather several) time pad methods are actually used in public key encryption for efficiency reasons, with the principal long-term protection of the public key encryption system only covering the sharing of a common symmetric key for a single session.