I am shooting with a Canon EOS Rebel X S and I am using Ilford Delta 400 professional B&W for the first time, any recommendations which ISO setting I should use? I have heard 320.
Each film has a personality that is further accentuated by developer and pushing or pulling.
If this is your first go around with Delta, I'd recommend shooting it at box speed (400) and seeing if you like it and then adjust from there.
The only caveat to that is if you're planning on developing in rodinal. I've found that films lose a little effective speed but I've also been doing stand development with rodinal as opposed to normal development. If you're going this road, then yes, exposing at 320 would be ideal.
But if you're going to use ddx or just about any other developer, go with the box speed and see how you like it.
I'm curious what's motivating your question since the film's name (Delta 400) is telling you exactly where to start. Camera is irrelevant insofar as it's capable of handling the range of film speeds you need. Developer, though, is not. Are your shooting conditions challenging for some reason (for example, low ambient light where flash is prohibited)? Do you need a particular kind of result (for example, very fine grain)? These would factor into your developer choice which, in turn, could factor in to the film speed you choose.
For general purpose shooting, I would recommend box speed (400) and a tried and true developer like D-76 or HC-110 (or equivalents). But read the fact sheet of whatever developer you're considering. Certain fine grain developers (Perceptol) and compensating developers (Diafine) work best when shooting below and above box speed respectively. But I wouldn't start with either of these.
For the first time pick something tried and tested - D76, ID11 or the like. Rodinal is not a great idea for starting with the tabular grain films (perhaps later).
Once you get the hang of it try something fancy. the Massive Dev Chart is a good place to start.
The idea with using lower than nominal ISO (first overexposing and then underdeveloping a bit) is to get more shadow detail. Negative film (unlike digital sensor) is highly resistant to overexposing. There is always a bit more detail in the highlights. Shadow detail on the other hand gets lost easily.
This is best reserved to more experienced darkroom technicians. Do the first couple films the plain vanilla way, and once you get the hang of it try experimenting.