As a first line I'd crack open the case again and reseat all the cards inside. Sometimes when you open the case and it hasn't been open in a long time just the motions of pulling it open will cause a card to jiggle a little loose.

If that doesn't affect the problem you'll probably want to switch in and out your RAM just to further narrow down the problem area. Have you looked at the power requirements for the new hardware you installed? Dell PCs usually come with just exactly the right power supply, with little room for expansion.