Actually, there are DDR GF4 MX 420 card's out there, but with a 64-bit DDR datapath, which doesn't make it any advantage to 128-bit SDR on the 'regular' cards, and in many cases, castrating its performance.
The newer Detonator 40 drivers (newest revision is 41.03) improved my performance significantly - you should give them a try, as well as using a prog such as Rivatuner to help with a bit of overclocking/tweaking.