ldopa has you on the right track on the SBF part. It is also imperative that you do research before you make any kind of big change to your phone. I know others have already told you but when I say it is "imperative" I mean it is no joke and no short-cutting it. There are way too many D2G users trying to do the same exact procedures as what DX and D2 users do and they can't be doing that.
While you are right that D2 and D2G are very similar, they are still different in many ways and require separate porting such as themes, ROMs, and anything else.
You also need to keep in mind that not ALL ROMs that work for D2 or DX is automatically going to work for D2G. So, you need to keep that in mind and check to see if the newer ROMs are compatible for the D2G (ported over). As far as I know, anything from Liberty 2.0.1 and downward will only work on the D2G with the help of Git R Done Rommer.
Before you even think about installing and flashing ROMs and themes you need to ALWAYS have a means of getting your D2G back to stock or any files/tools to help the process in going back or fixing a critical error.
So yes, before you get to flashing a ROM, research and get familiar with the process of rooting, process of SBF, process of using Git R Done Rommer and what ROMs it will support and anything else associated with what I mentioned. It also doesn't hurt to read through various threads talking about issues and such. You can really benefit from those kinds of threads and other threads talking about fixes, work-around, extra features, etc. That way you'll know if you run into something you'll be like "hey I saw a thread talking about this issue I'm having, I should go find it and solve my problem."
Good luck with the SBF.