Hi moh23. Just to get your posting to the top of the list where it has more chance of being noticed, I'll make a few suggestions. The expensive graphics cards are really for gaming where high power image processing is required to handle the 3D processing with high frame rates and pixel counts needed for the ultimate gaming experience and competitiveness. For DVD's and other less demanding roles almost any modern graphics card will probably be an improvement on your onboard graphics. I think if you are prepared to spend up to about £50 on a card you will probably get something quite good enough. This Fx5200 card from Ebuyer is an example of what you can get in that price range click here
If you can tell us what your present onboard graphics is, and how much you are thinking of spending, then perhaps someone can give more specific advice.
A bonus of changing to a separate graphics card is that you will free up some of your RAM, and so improve your Windows performance.
Download this program and install it.click here Then run the program and in the left hand pane click on the plus sign by Motherboard then click Chipset from the dropdown menu. If you have an AGP slot it will tell you the speed that it will run at.
To disable the onboard graphics, start up yuor computer,as the text comes on the screen it should tell you what to press to enter the bios normally it is delete or F2 key that you have to press. When in the Bios try looking under Peripherals for the onboard graphics. Then Disable it and press the F10 key followed by enter to save and exit the BIOS.
As far as your memory question is concerned you need to either read the manual if you have one or enter your type of motherboard into a search engine to look for the manual.