When your PC starts running low in available RAM it will start using page-file (assuming you didn't disable it). Page-file is not as fast as RAM as it runs direct from your hard drive, so you always want to ensure you have enough RAM.
I would strongly suggest getting at least 4GB of RAM but please be aware that system slow downs are not always a result of high RAM usage. WHen your PC starts slowing down you should open task manager and then the performance tab, as this will show you how much CPU, RAM and HDD are being used. If one is at 100% and the others aren't then you have found your bottleneck.