And also remember that to take fully advantage of those 8 gigs, you'd have to run Logic in 64-bit, so make sure that all your 3rd party plugins are 64-bit ready.
Otherwise, Logic in 32-bit will use less than 3 gigs of ram.
Kontakt I think has its own memory server, but I don't know the impact of that in 32-bit.
Logic Pro 9 Master Pro
2x2.26 Intel Quad-Core 6GB RAM, Logic 9.1.6, 10.6.8, Motu Ultralite