For me 8gb is plenty, but I don't try to multi-task lots of stuff while I'm working with Finale.  I also tend to work on 'shorter compositions' that rarely exceed 5 or 6 minutes.

Garritan Libraries are not very demanding on system resources, and with a decent modern rig alot of it can be streamed live from the hard drive as needed anyway. 

Unless you're doing pretty massive arrangements with more than 32 parts going on all at ONCE.....8gb should do OK.

If you can afford to go for 16gb, do it, you'll probably be glad for it over time (for things other than than just your DAW performance)......but for general compositions in something like Finale, using Garritan Libraries, it's not a necessity.

If you do LONG SCORES, as in multi-movement Operas, Cantatas, really long symphonic pieces, etc, and you're really hoping to keep as much of it as possible in a single file....then the extra RAM may well speed up the 'notation' portion of your workflow as Finale evolves more soundly into the 64bit realm of computing.  Not just for the audio side of things....but for the extra space for Finale to keep longer scores in memory.

So, for Garritan Libraries sake?  Not really necessary unless you do BIG arrangements with many voices playing at once.  For Finale's sake...in terms of laying out larger scores with less grind....it may well be worth it over time.