You will never receive a PR8 unless you really deserve, Hope upto PR6 after maxium efforts.
Most of directories submission is vein, because when you will get a categorised listing that page will either be a PR0 or maybe PR1/PR2 and none more then this, Better prefer getting less but powerfull listings like PR6/PR7 a single PR6 listing will work better then 100 PR2 listings.
For a forum, Article numbers doesnt matter at all, but whatever matter you have must be unique for better and good ending crawl.
PR will last for a long and in the meantime if you leave the domain to parking it will go down, but as soon as you put stuff it will come back.
Realisticity, About it I wish you yourself submit in some high PR directorirs, little will work, save those are high PR or direct links.
I also submit my website in alots alots alots websites and done alots link exchanges, But my page rank is not going up to 4. I got info from some one, That if we add Some good services in our website. And users come again and again then we will got more better page rank.