This forum is run by phpBB which is free.
There is another "lightweight" version called punBB.
Both worth a look in my opinion.
Also reasonably n00b friendly, as you have used phpNuke you should know a little about databases and the such like!!
But is phpBB also a portal style package, or is it "just" boards?
I need the portal aspect, and ability for member generated content etc.
just board: discuz!(as good as VB and free, pay for support, most pop in China) and Vbulliten(the best, pay for forum and support, most pop in US)
Portal....just use PHPnuke, it's good enough.....
i strongly reccomend php fusions i use that to my portal now, but i have moded it a littel bit but you can also donwload other big mods to. I acctuly tkae php fusion before php nuke