You'll never be able to access anymore than 3gb of ram and that may make it take too long to be worth your while. You will have to decide if you really want to wait that long for a render :). I have a quad core and six gigs of ram on a 64 bit system and it takes a good long time for Luxrender to finish a render. I don't mind it tho....overnight is usually more than sufficient in most cases ;).

Laurie