For Christmas I bought my brother a Zotac GT 430 1 gb card. Today when I installed it for him I noticed a couple things on his old card and now I'm worried I've somehow downgraded his video! His old card was a GeForce GTS 250 1gb.
Looking around at things the impressions that I've gathered are that the 420 is a (relatively) newer card with support for DirectX 11. The 250 is slightly older and doesn't appear to support DirectX 11, but may have been a higher performance card.