The amount of RAM you can add depends on how much your motherboard physically supports. Most motherboards typically have 4 RAM slots, but there are high end ones supporting 6 and low end ones that support only two. Looking up the model of your computer shows that it only has two RAM slots, and can only support up to 1 gigabyte of RAM. Unfortunately you won't be able to add any more.
To answer your question regarding Windows 7 and 64 bit, the specific operating system itself is not relevant, but whether it is 32 bit or 64 bit is what you want to look at. With a 32 bit operating system, you can only utilize around 3 gigs of RAM, whereas a 64 bit operating system can use any amount you can fit into the computer.
Regarding video RAM, that...