First of all you have to understand one (of many) key thing.
ICS is an upgrade to GB. There are ROMs that take GB components and make them look and feel like ICS, and there are ROMs that are based upon ICS
leaks.
Motorola is working on releasing (or else teasing us with false promises to release

) a true ICS Blur version for our phones, which will take our phones from 5.9.905 to 6.7.2xx. If you are interested in what a stock Motorola ICS upgrade would look like, I suggest that you check out 6.7.232 (the last ICS leak from which you can go back to 5.9.905, which is where you'll have to be to take the official release when Motorola an VZW release it).
Also, do note another thing - GB cannot run multiple cores - only a single one. ICS can run multiple cores. So, if you take a ROM that makes GB look and feel like ICS, it will only partially do so, as it still only uses one core.
If you take an ICS ROM that is based upon ICS, you'll need to apply the leaked upgrade first in order to use that ROM. 6.7.2231, 6.7.2233, 6.7.229, 6.7.230 and 6.7.230 are the safe leaks o install, from any one of them you can go back to 5.905. Anything higher and you're stuck on it, unless you use HoB - and even then, you'll never be able to go back to 5.9.905, which means you'll have to hope and pray that the final release is compliant with the current latest release of 6.7.244, so that you can use HoB to install it - else you'll be semi-permanently stuck on .244.
I know I threw a lot of terms at you, and this is a very small portion of what you really
should know before going any further, but essential knowledge to your exact question.