Well the 2 I've heard most about are the following;
1. Asus 4200-128MB Deluxe, default clocks 260/550, o/c 300/640. I'd expect that to be a minimum for this card, 3.3ns BGA should go higher.
2. Suma 4200-128MB Special Edition, IIRC default clocks about 260/550, o/c 300/700. Since they don't seem to sell these in the West I haven't heard much about them.
So to answer your question, it depenmds upon the precise implimentation and as with any o/c'ing the luck of the card to get the top o/c, but about 300/650 should be doable with at least 95% of 4200 cards using 3.3ns BGA.
As I mentioned 4400 cards can be had for about the same money as the 3.3ns BGA 4200 cards, they come with 275/550 defaults and o/c to around 300/650 and have the added benefit of selling on better when the time comes too.