You may try first to reduce the space and bandwidth to save on moving to a higher hosting level.
Try to get a behind the scenes look at what's taking up space and bandwidth.
Some things to try:
1. Optimize images, the larger images like maps that are large and popular suck more bandwith than txt. Html pages are typically much smaller.
2. Try disallowing the image bots and link walker bots in your robots.txt. This won't affect traffic.
3. If you're html pages are too big, try dividing the information up on multiple pages.
4. Purge logs after a certain time limit. This can be a big space sucker. Take the important info each month and print it out or transfer it into a excel tracking spreadsheet. Then have it deleted every 90 days... if that's more than you need, do it every month.
5. Check your mail logs. I noticed I had a large collection of mail multiplying from accounts I never set up. Make sure any email that you aren't using is being deleted to a blackhole.
Hosting can be really cheap once you streamline these main parts. Good luck!