The clock speeds on these cards are pretty much identical, (depending on whose card you buy) but there are a few reasons why I might be inclined to go with the 9800 NP, if I were in the same situation.
1) The added features of the 9800 core MIGHT make it better for future games. This would mean very little to me if the card were more expensive, but since they are the same price, I think it would make since to go ahead and get the 9800.
2) The 9800 PRO can overclock much higher than any 9700 PRO ever could, and I see no reason why the 9800 NP would be any different. If you like to overclock, I think the 9800 would be the better choice.