Overload, CPU or RAM?

Using a MacBook Pro atm, and I was considering upgrading to 8GB RAM, since audio starts acting up when using too many VSTs at the same time. I suppose my question is, if this is in relation to RAM or CPU?

I can easily upgrade from 4GB to 8GB. The CPU is i5 2.53GHz 2 Cores HyperThreading. I also have a Native Instruments Audio Kontrol 1 USB SoundCard connected to it, perhaps the latency settings on this is relevant?

Activity monitor:
http://i.imgur.com/wy88K.png

Increasing the buffer size on Audio Kontrol 1 seems to help, but the audio latency is about 15ms, which is a little much for playing piano etc. Is there some way to enable a hotkey to change between the two?

Edit: Not by much though, eventually overloads and crackless regardless…