I´ve recently formatted my laptop and upgraded it to Window 7 Ultimate 64bits. My laptop is supposed to accept a maximum of 8GB RAM. I was wondering if there is anyway to change this via hack/system change so that I can install more than that.

There is a a hardware limitation on you laptop for the amount RAM. Also, the operating system you use will limit your maximum RAM. I am currently updating my PC to be dual boot. With Vista 32-bit I am limited to 4GB of RAM that can be used. With Windows 7 Home Premium 64bit my PC will accept up to 16GB.

Memory Limits for Windows Releases (http://msdn.microsoft.com/en-us/library/windows/desktop/aa366778%28v=vs.85%29.aspx#physical_memory_limits_windows_7)

I´ve just been told that this is a motherboard limitation. So I´m stuck with a maximum of 8GB.
BTW, right now I have 6GB...is it worth it to upgrade to 8GB?

One more question. Is it worth it to change the virtual memory size after I´ve upgrade the RAM to 8GB? I have a hybrid HD (Momentus XT 750GB) as my main HDD.

Premiere Pro is RAM hungry
In my own experience, 8 GB was good, 16 GB was better, 24 GB is actually better yet.
The need for RAM will obviously vary with the type of project, effects, layers, formats, etc., but it clearly seems that more is better.
RE virtual memory- it is supposed to be sized in relation to the amount of RAM onboard.
So, if you go up in RAM, you should, in theory, increase the virtual memory.
I don't know for a fact how much this would actually matter for using PPro.

Get the ram, every drop is worth it. Leave VM setting alone.