Hi You will probably need to disable the on-board graphics adapter in the BIOS.
When you switch the PC on look for messages along the lines of "Press Delete for Set-up" or some such. It could possibly be Delete, a Function key, or another system key to enter the BIOS.
(If you can find out the exact MOBO and BIOS revision you should be able to find out how to access the BIOS on the net).
Once in there you should find a section that contains details of the graphics adapter and the memory it uses.
Hopefully somewhere there you should be able to disable the on board graphics, then then when you restart the PC should pick up the new hardware!