If you've already got the two sticks of ram, then I don't see any reason to swap them out unless you're planning on maybe going up to 1gig sometime.
As to a new computer and running dual ddr, I might be the only one who doesn't like the idea, and here's why: sure I totally agree that with a given cpu & mobo, dual ddr is faster than single channel ddr. However, instead of buying a dual ddr enabled board & two sticks of ram, the money saved by a cheaper board & one stick, you've just upgraded the processor a couple speeds which overall is better on performance. The price gaps between P4's are a bit more than AMD's, with an AMD setup, I once figured that dropping from an Asus nForce2 mobo to an Asrock SiS748 w/ single channel, the price savings upgrades the Athlon from an 1800+ to a 2600+!!!
I'll stick with single channel.