For my sins I have a PB x6719uk, having a M3720 motherboard. Will adding more memory ather than the 4gb present 2x2, give it a noticeable difference?
Also, can i put in 1 x 4gb and 1 x 2gb ti its only two slots?
Well, it depends on your operating system. If you are using Vista or WIN7 64 bit then your system will use the memory when under big workloads or a lot of multitasking.
So it also depends on what you use your computer for. I find my machine runs better with 8 gigs than it did with 4, but I am a power user.
When adding ram it is always a good idea to match the current ram exactly, same maker, same model, same capacity. Otherwise its likely you'll run into compatibility issues. Sometimes it just makes more sense to buy a whole new kit and sell or re-purpose the old kit. For instance right now 8 GB ram kits are going for as low as $30 here in the US. Buying that is a smarter idea than paying the same amount for just 2 or 4 gigs of ram.