Is it possible to install Gigasampler 3 Orchestral into a PC which contains only 512MB of Ram?
It's possible, but not really advisable. I would also imagine that the CPU would be too slow if that is the basic spec of the machine (although this is just a wild guess). You must remember that part of the RAM goes to the operating system, so that what you have left to load samples with is very small.
I'm only running v2 but I'd be surprised if Gigastudio 3 itself requires much RAM. The question is really: How big are your libraries and how many of them do you intend to load at one time? It's like a water tank. You just keep adding water (samples) until it's full. You'll run out, even if you have 3 gigs - it's just the nature of the beast. But fortunately you'll gain experience as well as accrue sounds as you fill it and buying more RAM is not all that expensive and just part of the growth curve.
In a way, it's a closed system. If you are asking about using just 512 mb of RAM, it's not likely that you'll be purchasing huge and therefore expensive sample libraries. (TBO NS sale being the exception, of course. My guess is it's the cheapest dollar-per-mb-of-samples library in existance)