The 420 should have its own ram on board. Those Dells usually come originally equipped with onboard video chips that share the system ram, and I assume you added the 420 later. When you select the add-on card to run the display, I'd guess the old onboard video chipset stops running, and hopefully stops taking from your system ram. (I hope this is the case, because I'll be adding a 440 mx over my onboard video tomorrow.)