Everyone shouldn't post if they don't know what they are talking about.
Like the guy who suggested an 8400. I see no point in 'upgrading' to a card that was mid-ranged a few years ago.
If your budget really is $420, there will be some great deals you can get, we just need your mobo.
Currently ATi is best performance for money, although nVidia may be better quality in some cases. I personally have had 2 bad experiences with their drivers.
I'll recommend some cards once I know what it can take.