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.