Basically from what i know, if your bandwidth is bigger than DoS bandwidth, forum should be fine, depending on how big attack happens (100+ GBps happens now). But relying only on that is not enough in my opinion.
Also there are many companies providing DoS protection (Verisign, defense.net, Prolexic, Akamai) which should have enough capabilities/trust to be used as the last option.
I would recommend reading
http://www.itbusinessedge.com/slideshows/show.aspx?c=96534&slide=7 to make some entry point on this problem and then you'll have to look for some solution that would fit the best here.
Unfortunately this is solely only my opinion, and i am not network expert, so i hope someone with more experience than me will also contribute.