That's what I thought. But it gets more interresting.
If you have a socket A board and a CPU that supports 400fsb (200mhs * 2). The the fastest RAM you can use would be DDR400. Right. But if you put that DDR400 in D.C. mode, it can technically transfer at 6400MB/s, or twice the 3200MB/s. However, since the CPU is still limited to 400, this gives you no gain.
But then lets say you're on a budget, and you have 2 sticks of PC2100, or DDR266, this is only running half the speed your CPU supports, but if you put 2 sticks of pc2100 in D.C. mode, it would raise the speed back up to the 400 speed, thus running 2 sticks of PC2100 in D.C. gives you the same performance as running a single PC3200. Unless I'm wrong.
Lastly, as pertaining to my original question. Athlons are just to expensive anymore to buy, but the problem is that all these Semprons use 333fsb for some reason. Only the 754 versions seem to have 400fsb support. So then I am thinking, if I buy a DDR400 stick in single channel, my RAM is already going faster then the 333 CPU can support, thus the speed is already wasted, regardless of D.C., in which it would be faster, but have no benefit at all.
Oh ya, and I wasn't mixing up D.C. with DDR. Just saying that, your CPU's fsb speed, such as 100mhz, is doubled out the door to make a 200mhz FSB. Thus matching perfectly to DDR266 (PC2100) RAM. BUT, D.C. doubles that rate yet again, thus giving your RAM the speed of a 400fsb CPU. However, since the CPU fsb is limited to 200 anyway, the D.C. would not give any gain. Which makes me wonder if running D.C. at all on AMD chips ever did have any benefit? Assuming your already had RAM whos speed matched the CPU's fsb.
This all comes to nothing anyway when I realize the Sempron has a 333 bus, so D.C. would mean nothing if I have DDR400 RAM.
But it's to late! I already bought a 1GB Corsair kit last night. Doesn't matter anyway, it's 1gb either way I configure it. And it was a dollar or two cheaper then the single 1gb stick.