Lack of RAM is primarily the main reason why your PC would run slow. There are many kinds of RAMS that can be introduced into the system that would enhance their performance.
For a layman RAM can be explained in a manner by drawing an analogy between a computer RAM and an office desk that most people utilize as their work station. The hard drive is the file cabinet, the bigger the better for your utility. If you put all the files in the filing cabinet then it would not clog your daily routine at all. At times you might need to pull some files from the filing cabinet to function effectively. In that case you might have to put these files on the table top along with your tea or coffee cup and at the same time you might be listening to some music as well. Also you would have some stationery and pads in the table to add to the utility of the work space. This entire scenario can be replicated in the computer arena. RAM stands for Random access Memory, which signifies the space that you have to work within.
This can be similar to the table top space that you have, therefore in case of RAM, the bigger the better for you. The more functionality that you are running the more space or RAM would be required by your system. This is similar to the fact that if you want your table to do multiple functionalities; that is to accommodate for your files, stationery, your music system and your drinks; then you need a bigger workspace to accommodate your growing needs. The same goes with your RAM capacity as well, in case you just want your system to perform a plain simple function then less of RAM is good, however in case you want the system to have the ability of multitasking then the more RAM that you have in your system the better it would be for you.
RAM is such a standardized thing that if you buy RAM for one system it is more or less compatible with all the other system with few exceptions that the RAM vendors would be able to tell you about. There have been many kinds of RAMS that are available in the market today. Most of the systems come with DDR2 SDRAM 256MB modules. However after SDRAM DDR RAM was introduced in the market, the difference between the two is intricate that they are actually meant for only technical people. However there is one word of caution that is to be kept in mind; which is that DDR RAM is not backward compatible, implying that systems that are made for DDR RAM cannot work where SDR RAM is required in the systems.
After DDR RAM, DDR2 RAM further marks progression over the existing RAM. This implies for a faster transfer rate of data with lower consumption of power and lower voltage operation. Another advancement of DDR2 RAM over DDR RAM is that it led to lower heating of the system. Again this RAM is also not backward compatible. After this DDR3 RAM was introduced this further marks a progression over DDR2 RAM. This allows for three times the transfer rate of data which certainly is a progression over the other kinds of RAM. Therefore if you feel that your system is running slow then RAM upgrade is certainly the way to go instead of buying a new system. And certainly this is a more viable option as it would not only enhance the performance of the system but would also cost much less than the purchase of a new PC.