The speed of your computer depends on a lot of things; RAM amount is one of them. But there are more considerations to it than just how much you have - if your system runs out of RAM, it will page to the hard disk instead, which makes it slower because the access time to your hard drive is significantly higher than the access time to the RAM.
Upping your system to 6GB of RAM might work, but only if it is a 64-bit operating system, with a 64-bit CPU. If it's 32-bit, the system can only address 4GB of memory due to the limitations of the architecture, meaning the other 2GB will be completely wasted.
Windows 7 comes with a light benchmarking tool in the control panel, under "Performance information and tools". There you can see a rough idea of where the bottleneck in your system's performance is. You can find out quickly if your system is a 64-bit operating system by looking at the C:\ drive - 64-bit versions of Windows come with two Program Files folders, one called "Program Files" where the 64-bit applications are kept, and one called "Program Files (x86)" where the 32-bit applications are kept. A 32-bit Windows installation only has "Program Files".