To go dual channel, it's highly recommended that you run 2 sticks of the same size, same speed, same brand, same everything memory. Dual channel on AMDs is pretty much a waste and the single channel board is quite a bit cheaper with pretty much the same performance. If you're not overclocking and buying a 333MHz FSB AMD CPU, your PC2700 is fine. Unless you sell your PC2700, it's not worth upgrading to PC3200.
So yes, for the sake of price, just keep your RAM and get the -X version. But you won't get that nice nForce 2 sound that the -DX (deluxe) version has. Which may or may not justify the price for you.