Yes, I suppose it is optional. But its use just emphasizes that the questioner is asking about the remaining available memory on the computer, not the total theoretical amount. Depending on the circumstances, it might be necessary to remind the person being asked the question to check how much is available and not just assume there is sufficient to do the job.

Do you have enough (available) memory to run the program? If you drop the word "available" , the meaning changes very little