If you are running Windows XP then 224Mb of memory is NOTHING.
I would say 512Mb at least, even 1Gb.
Many cheaper computers have "on board" graphics. This is graphics built into the motherboard and it usually uses part of your 224Mb of Ram (so you have even less RAM).
Buy a graphics card with its own memory, 256Mb or 512Mb.
I am afraid I know little about graphics cards, but I am sure someone here will help, or go onto a special games forum. (there is a Gaming section here on answerbank)
You will need to know what "slots" are included on your motherboard to take a grapics card, either APG or PCI. This will determine which type of graphics card will fit.