Read the data sheet before you gather too much...
It is an ISO 1000 film with Ilford's methods of testing. Notice on the package that it is NOT labeled as "ISO 3200", but only uses EI 3200 as part of the proper name for the film. IMHO, they should change the name to Delta Super Speed or something similar to avoid all the confusion. There must be one post a week asking the same basic thing about this film.
So, it is ISO 1000, but Like all films, you can get usable results at a variety of ratings, with certain trade offs, of course. It follows the general rule of thumb for films: the higher the speed, the lower the contrast, the more grain, and the less sharpness and resolution you will get.
The great thing about this film is that it allows you to get lots of nice, neatly-patterned grain without giving you too much contrast.
If you want a more grungy, clumpy grain, I would overexpose HP5 by about two or three stops and give it development in a weak dilution of a harsh developer for an extended time, at a high temperature. When I shoot for gnarly grain, I rate HP5 at 50 or 100, and develop in highly diluted D-19 at 75F for about 12-15 minutes. I often have to bleach back the negs a bit to make printing easier.
My zone system EI for this film in my C33 with the 80mm lens is 640...but that is for my particular setup and developing methods. Very rarely will I shoot this film in a situation in which I will be using the zone system. I find that, in non-zone system use, it needs to be rated at at least 2000 to get a "normal" looking amount of contrast in a "normal contrast" lighting situation. I usually rate it at 4000 or 8000 in a low-contrast lighting situation. In the situations in which I *most often* shoot this film, I don't give one hoot about shadow detail. All I care about is from about zone V on up.
I would start by calling it 1000, and see if it has enough contrast for your situation. If not, rate it higher the next time you shoot it.