Your Intel D915GLVG has a integrated
PCIExpress built into the board. Your system memory is shared with the built in video card. This board's memory can handel up to 4GB of system memory which up to 512MB can be shared from your system memory. If you wanted to disable the integrated
915GL Express Chipset with Intel® Graphics Media Accelerator 900. You would be left with two PCI slots. Running a PCI video card is slower than a AGP card & two notches down from a PCIExpress
So in order for you to acheave what you want your going to have to increase your system memory. If you still have your manual the specifactions for your memory is on Pg. 9.
The link below is a list of supported memory from Intel.
If you no longer have your manual you download it from this link:
Though you can check aound. I can suggest this mamory which will give you 2GB dual matched memory that is within your boards specifications. Listed at this link. http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=1075233&CatId=1352
You will still have two memory slots free to upgrade to a total of 4Gb at a later date. So please do not disable your current video card. Please use your manual when installing memory. Two Dimms go into certain slots on the D915GLVG board.