Ram Memory for VST synths

I have a Power MAC G5 2.5 quad core and have had problems with not enough memory for a vst synth ( Native Insturments Symphonic Choirs)
I had 4 gig of memory and put another 2 in to bring the total to 6 gigs, and when I load some of the synth settings it says memory is low and many of the settings. This is just one instance of a bank. This vst does eat alot of memory but 6 gig seems rather extreme. Is anyone out there dealing with this problem and if so have much memory do you need. Can I mix 6 one gig memory chips with 2 more 2 gig memory chips to make a total of 10 gigs or should it be all 1g or all 2 g chips. I have a total of 8 slots on this machine.