I only have a 1GB memory, so I would do most composing with Garritan's and then replace all the parts one-by-one with EWQL when I do the final rendering.
I have 2GB mem, but have to do the same thing. It seems to be more a processor thing (I have a humble 3GHz Northwood core P4) when you get multiple samples going in Kontakt. Painful trying to mix, as I have to freeze the synth to audition each change, then unfreeze to make another change, then freeze to audition...
I never used Edirol Orchestra, but have heard many good reviews.
It is nice. Still, when you get good samples in to replace it and spend the time tweaking, it makes a big difference to the end result.