If I were you I would start with shared hosting. Then as the time goes by and if your forum outgrows that then start looking to upgrade it to a vps/dedicated box.
I love hawkhost used them for 2 years going on my third. You can find a 50% off coupon on webhostingtalk.com (i think its still there) it'll get you 50% off the first bill I prepaid for a year at the basic was only 18. Price goes up after first year, also there support is friendly and quick too
For our needs we ended up simply buying their resell plan. This gave us the option to generate accounts and domain names at will (great for other projects).
$189 for the year was not a bad deal at all. With their current special of 25% off (coupon code: rseptember2012 ) it comes down to $142 for the year at Hawk Host