You haven't told us what OS are you using.
Anyway, yes, it will speed up your computer giving your applications consume more than 198MiB RAM. Because of extra consumption, memory is 'cached' in a swap file, which only adds extra I/O cycles to the HDD. By adding more RAM, less application memory will be loaded into the swap file, and more directly into your RAM, which will speed up your system.
* Note that some 32bit OSs will not let you see the whole 4 GiB RAM (Windows XP, for example, can only see aprox. 3 GiB). If you're not planning of covering more than 3GiB of RAM, for Windows at least, you will see even a greater bump in responsiveness if you disable the swap file/page file all together.
** Make sure your system support 4 GiB of RAM! Older systems may only work with 512 MiB DIMMs. Check your motherboard manual over the Internet, if you don't have a physical version of it.