Ask HN: Purchasing Protocol Advice

I'm starting a new job soon, and I've been given a budget to spend on what will be my laptop. What is the usual protocol for this? Can/Should I take the budget they give me and perhaps add a couple hundred euro of my own money to get a better machine or stick to the budget and get the best that money can get me? I know asking my boss is probably the best idea but before I do that I want to get some opinions - asking about these kind of things when starting is a little intimidating. Thanks!