You shouldnt have any problem installing Win 7 on your laptop due to the disk size. The 320GB HDD is more than enough to cover the windows install and still have plenty for storage left over. If you're on the market for a 500GB disk I would suggest picking up an external disk. It would be more handy for everyday storage and stuff, plus you will have the option to use it on other devices too.
The only thing that may be an issue is if your laptop is an older model, you may have driver issues. But thats a big "if", Win 7 is very good with hardware. I have installed it on some pretty old andd crappy pcs and had very few problems.