Ok buddy, here's the deal. I want to make sure you understand what you're going for.
AMD CPUs don't have a conventional FSB, but rather something called a "HyperTransport" bus that has its own multiplier (5x in this case) and is also double-pumped (think DDR). That's why the CPU is advertised with a "2000Mhz HT": 200Mhz (base) x 5 (multiplier) x 2 (double-pumped) = 2000Mhz. An Athlon64 3800+ runs at a stock 2400Mhz, which now you see as being 12x200Mhz.
Now, the multiplier on the CPU is locked, so what you do to overclock is increase the base HyperTransport (HT from now on) frequency, ie: make it higher than 200Mhz. This will overclock your CPU and
also your memory. So when you're using slow or value RAM, it will be the memory that limits your ability to increase the HT any further.
So what you're looking for is memory than can run at more than 200Mhz. DDR-400 is designed to run at 200Mhz (remember *real* speed is half the DDR advertised speed). If it's value RAM, you won't be able to push it much further than 200Mhz. On the other hand, DDR-500 is designed to operate at 250Mhz. This means you'll have at least 50Mhz of room to overclock the HT bus.
If you install DDR-500 RAM with your Athlon64 3800+, you'll notice the motherboard will set the memory multiplier to 5:4, so that it can run at its designed 250Mhz (200Mhz * 5/4 = 250Mhz). Here you have two choices: just increase the HyperTransport frequency and hope the RAM can go beyond 250Mhz - OR - decrease the memory multiplier to 1:1 (memory will now downclock to 200Mhz) and raise the HT ... this way the memory will only start limiting your overclock when the HT goes past 250Mhz (already a 25% increase), which would take your CPU to 12x250Mhz = 3Ghz, if your motherboard can handle it.
Edit: on reviewing a few 939 Asrock mobos (since I don't know exactly which one you have), it seems the previous paragraph may have some errors. There's no memory multiplier; rather, it's a divider. I forgot DDR and older motherboards usually have dividers. So there's no 5:4 setting (the maximum is 1:1, and it goes down from here). So I'm guessing DDR-500 RAM will automatically downclock to DDR-400 speeds. This is not bad if you're overclocking though. I would've manually set 1:1 anyway.
Yet another choice would be to get high-performance DDR-400 memory, hoping it can be overclocked enough. I guess some DDR-400 modules can be overclocked to 250Mhz (DDR-500 speeds). Here
they overclocked a Athlon64 3500+ to 3Ghz using Corsair PC3200XL (DDR-400) memory. The final HT speed was 277Mhz!
Try borrowing high-performance DDR-400 or value DDR-500 RAM from a friend, in order to determine how far you can push the HyperTransport bus. Then
you'll know exactly what memory you need.
Lol, that came out a bit long. Hope I'm not confusing you too much.