I rarely use Garritan anymore, but when I did, I had less than a gigabyte of RAM and it stell ran pretty well unless I tried to load a full orchestra. And what Garritan means by a full orchestra is literally loading 2 clarinets, 3 trumpets, etc.; exactly the number of instruments you want, rather than loading just one of each. So really it's not as heavy on RAM as they make it out to be. Well, depending on what quality you're going for. But I always had one sample of each instrument loaded (and the brass requires overlays) and it sounded pretty good.
tl;dr: You'll have more than enough RAM to run Garritan.
Garritan does have a few drawbacks though, as far as quality.
*The oboe sounds like shit.
*Not much percussion
*The percussion sounds like shit.
*French horn isn't that great, but it's alright.
That being said, the piano and solo string instruments are beautiful. Though, you may want to think about someday adding this (
1|
2) for more percussion. Obviously if you did that would be further down the road considering the high price, but still.
Yeah, uh, that's all the help I can offer. :3