Since this is a few days old, I'd like to give a bit of advice.
First off, there's no problem whatsoever with making a GTA Forum as long as you make a original name up, but make sure it sounds catchy, that's how you get some members to join up. Secondly, You're thinking of buying a IPB License eh? Well good luck with that as it cost's alot of money, but if you do get it eventually, you'll see how good IPB is for you. Thidrly, when you purchase IPB, you need a bit of knowledge with a FTP Client which you use to set up the files in your server/host. Instruction's will be setted up inside your documentation. Fourthly, You must be good at designing, so making skins for your website will be the trickiest of your job (Trust me on that one, I've skinned before). If you want your site to go big though, you might want to go further and make a news page which is tough to do. Best to download a CMS Script and place it in your host, then the design part comes again.
Also, make sure that you're on their at least a few times a week, you don't want a site without a active webmaster, do you?' I for one run a site, but it's not the best site ever as I'm trying to improve it. So whenever you start a website, make sure you know some basic stuff about FTP Clients and Web Servers, sometimes it's confusing, sometimes it's clean and simply, but you must be at the top of your standards to give what your members want out of the site.
Gerard has a good point too, I believe TGTAP is good site too, but starting a site isn't as easy. You might want to stick around here for a while and get this place improved, by posting some articles related to other games and GTA, help members that are stuck with modding or missions etc.
Well, all I can say is good luck to you Ice. (: