I agree with Diggsey. People wont pay a cent to be a part of a forum or any website that doesnt offer them something of equal or greater value for their dollar.
I also think that people that want to market their game will not go to such a place, because lets face it, you can get your own domain name for $10 to $12 per year and find really cheap (or free) web hosting servers and target any audience you want (with yourself calling all the shots).
The only way I see your idea working is if you've created some type of partnership deal with some major/well known company that will direct traffic your way and draw in many people to your site.
Dual-Core Pentium D @ 3.00GHz, 1.0 GB nVidia GeForce 8500 GT, 20" Wide screen LCD @ 10,000:1 Contrast, 4.0 GB DDR2 SDRAM, Triple-boot: 32-bit Windows XP MCE, 64-bit Vista, 64-bit Linux Ubuntu