Extra RAM is only worth paying for if you'll make use of it, and nowadays you'll basically only make use of it with lots of high-res textures and 4xAA at 1280x960 or higher. So on the literally handful of games that can go over 128MB without AA, it's worth it. If you have to use AA to tip you over 128MB, you'll want nothing less than a 5900 or 9700 using that memory, as lesser cards just won't have the graphical cojones to both read enough bits (256-bit is a must) and move enough pixels (4x2 or 8x1 is a must) at that stiff a setting.
DDR II is improved over DDR in that it's supposed to clock higher, but currently DDR is clocking almost as high as DDR II. There's nothing "better" about a card with DDR II over one with DDR, if both are the same price and speed. 128MB of DDR II would generally still be preferable to 256MB of DDR is the former were clocked much higher.
Crazily enough, 9600XTs in England are showing a ten pound price difference between equally-clocked 128MB and 256MB versions! That's one case where going for more memory is a no-brainer.