Yes, you can send email without prefixing some ones name. This doesn't typically account as spam but, it really does come down to the rules of the spam filters! So, treat my "Yes" tentatively. I send many emails to friends and typically don't start it with a Dear Matt or Hi Sara etc and they get the mail fine.
However, according to legislation (and I've read the US and European) the general rule is this: If some one has contacted you through normal business operations (eg, send you an enquiry etc) or they opt in, you can send them an email without making it personal.
I wrote a mass mail program and we never personalised it and we got responses. Whether it went into some spam folders or not; who can tell. I would assume yes! However, that could be for many other reasons.
However, as Alex points out, when something is personalised there maybe a higher chance that they read it (although this is off topic and also depends in if they view the email straight away or only the subject line).
My attitude towards this differs from many, but I would continue as you are. Get as little as possible at first; Don't put them off by big forms (And I'm not saying 1 extra fields makes it 'big' but it does make it bigger), get a method to contact them! Then, get a strategy in place on how you are going to fill in the gaps if you need it! Maybe the email sign up also automatically creates their account on your system. Then, after they log in again, you can have a little notification on the side saying "update your name details" etc. You're not forcing them, just offering. This is becoming quite a nice UX approach, it's more subtle and less intrusive on the user's visit.
Lastly, don't forget what spam you receive and how it is laid out. If you don't get any, I seriously suggest you get yourself a free account and sign up to legit good quality websites/services and just take note on the sign up process and how much of that detail is used on the emails you receive.