Your right some do eat up a lot of system resources. But Ikonboard from everyone tells me and knowing myself. It doesn't eat up as much as all the cgi boards out there that i have seen. I run a Ikonboard on my site and it's awesome works great and the community that comes along with it is great also best I have seen aside from UBBs community.
The best boards to try are the most popular ones.
>Ulimate Bulletin Board (www.infopop.com)
>vBulletin (www.vbulletin.com) *req. PHP/mySQL*
>IkonBoard (www.ikonboard.com) *Highly Recommend*
UBB and vB both have free versions of their boards BUT their limited. Ikonboard is and always will be a free board. They promise this! Also it's full of features and it keeps on growing.
For remotly hosted boards. You can try coolboards.com, ezboards.com, and looking up on a search engine "FREE BULLETIN BOARDS" or going to a cgi directory site such as http://www.cgi-resources.com OR http://www.hotscripts.com and look under their remotely hosted section.
I hope this could help you out!