Use only the new one first. IF it works, then you can try each leftover slot one at a time. Most systems with 3 gb of ram have 2 512mb sticks and 2 1 gb sticks, so each pair matches. You're trying to run with just 2 slots, but the sticks don't match. Dells and other oem systems aren't designed for upgrading. Their bios' has fewer adjustments. They'll sell you some new dell ram for about double the regular price. They have to make most of their profits on upgrades from the factory. Few people will purchase the ram later when they compare the price.