as i am planning to make it a forum, i guess after building some good members base and good number of posts, at that time my PR will pop me up in many search results at search engines, i guess Adsense will do the job also other some affiliate programs on this or that side will do a good job right? you have any more ideas?
It's really depends of forum niche, depends which you chose. In theory this is very simple, but reality is a bit random.
took me 3 years for one site i was actually trying for, another site tool about 5 years (but i wasnt really agressivly linking it)
I was wondering how long it will take to get to a PR 6 from a PR 4 if I buy a link or 2 on a PR 8 website?
Depends on the strategy you want, if you want to get ROI from selling links, they would get ROI for that. But in SEO, you can get ROI from the products and the clicks you have obtained from your site.
That depends on how much you paid to get there as well as the time frame I've heard numerous places which are good sources that until you get a high PR it doesn't look natural to have links from such authorative sites. (Unless a directory of course). You'd be better off to buy LOTS of PR 4 links etc maybe some PR5's and a PR6 or 2 but a PR8 just doesn't look natural. If you want this long term then I'd say make it look as natural as possible from the SEO point of view.
"it's all about the money" like the song google makes pr update every 3 months, so just buy 7 pr4 links of 2-3 pr8 links and your site will be pr7 in the next google update
Read lots of stuff about SEO , follow what Google says and consult a SEO professional .. Cheers ! Go through my thread.. http://forums.digitalpoint.com/showthread.php?t=276966