I am about to meet someone from HR to discuss a full time offer. Am I
in a position to be able to negotiate salary much? I had (sort of)
already agreed that I was ok with X amount.
I'm not sure what "sort of" means in this context, but if you sort of already agreed to an amount, then you can still negotiate, but the likelihood of getting what you want is probably sort of lessened.
Before you meet to discuss a full-time offer, consider your alternatives, and be ready to move ahead depending on how they respond:
- You can accept X
- You could ask for more and settle for X or more
- You could ask for more and leave if you don't get more
If you are confident that they like you, and don't suspect that they will react negatively, there would seem to be little downside to asking for more (perhaps asking for "typical market value"). Make sure you consider other aspects of the job beside salary - benefits, opportunity, market, corporate culture, etc.
Be enthusiast. Tell them how much you like working for them. But tell them that, while you initially believed their entry-level salary was reasonable, you have come to understand that the "typical market value" for someone in your position, and coming out of an internship, is more than they have offered.