When should I get business cards and how much do they actually matter as a graduate student who has not yet built up their CV?
There's no harm in getting business cards, though the importance in academia is smaller than in business contexts.  While you can give them off pretty indiscriminately, they're most useful when you've actually been having a conversation and want to carry on offline.  Therefore, your email address, phone number, website are the most important items.  Your official title doesn't much matter at this point in your career, in my opinion.

In the US, you can put together a simple business card and get a few hundred printed for around $10.