Dell Precison 360 and RAM

I got this Dell with 512 and went to bump it up to 4GB. It accepted it but only recognizes 3.47 – 3.62 and is constantly freezing up now. When I run magic speed it only sees 2024 and no ram in use. If I pull two gigs it seems to run fine. The ram is new and matched. Am I missing some configuration settings?????