Windows 7 Clean installed Windows 7 Ultimate 32bit from Vista Home Premium 64bit- How can I make 7 64bit??

Hello! This isn't making sense to me and I'm hoping one of you can help! I have a fairly new laptop that came with windows home premium 64bit. I got windows 7 ultimate 32bit free from my school, and decided that I wanted to clean install, and not upgrade it. I transferred all my files off, booted the laptop up with the windows 7 disk, and proceeded to clean install. I didn't have the drivers cd for my laptop, but I just wiped my drive and installed 7 anyway. Now I have windows 7 ultimate 32bit, and can only use 3 of my 4 gigabytes of memory.. The thing I don't really understand is the whole upgrading option, and like, I know that it's a windows 7 32bit dvd, but it doesn't seem right to go from 64bit to 32bit. So is there anyway I can make it 64bit without having to buy a new dvd? And if not, how much am I missing out on, with not having 64bit, especially with having 4 gigabytes of ram?? Thanks for any help!