welcome to the site. I am relatively new to this website, as it seems you are also. I spent many months just reading the website and hundreds of threads before I ever posted anything. that said....
JUST A QUICK WORD OF ADVICE: your question can possibly be interpreted in such a way that one might think you are asking how to obtain a digital copy of N64 games (ROM's). NOT ACCUSING, JUST SAYING. this forum has VERY strict and specific rules on asking questions about obtaining games (ROM's). although you didn't specifically ask about OBTAINING said files, you are kinda walking close to the edge, so just make sure you read through the rules of the site if you haven't done so already. here is a link to the RULES
now on to your question....
I'm guessing that since you posted this question in the UnleashX category, that you are running UnleashX as your dashboard. I too am running this dashboard, and I have transfered many many gig's worth of data to my Xbox from my computer. to do this best, you must install an FTP program on your computer, connect to your Xbox through the FTP program, and transfer your files. this is also useful to transfer files FROM your Xbox back onto your computer. a good FTP program for PC's = FlashFXP, for Mac = FileZilla. below I have listed a couple extremely helpful links to threads on this website that will help you to do all of this properly:
How to Transfer Files to and from Xbox HERE
Very Helpful Question/Answer Threads HERE
please don't take my reply the wrong way, I'm not trying to talk down to you or tell you that you did something wrong, I am only trying to give you a couple pieces of advice since this was your first post, and you may not have been aware of the forum rules.
your Xbox sounds like an awesome machine and I hope it brings you as many good times as mine has brought me and my family. This forum is the BEST resource for all things Xbox on the internet, so you came to the right place! Please reply with any questions, I'm happy to help.