I have PC2700U RAM currenly in my computer (This is from the sticker on the physical RAM).
From the Manual: The RAM is 184 pin, and the computer’s manual says that the computer is compatible with PC2100 and PC2700. For whatever reason, the manual also says that my FSB is 400/333/266. The last suggests PC2100, the middle 2700, and the first PC3200.
Is the unbuffered stuff rare? I take it from the link I must buy the exact brand of RAM that’s already in there. I usually buy from www.NCIX.com, and they don’t list PC2700U.
The motherboard manufacturer can give you recomendations on the ram to use. The ram manufactures can tell you which of the ram they make works for your specific computer. Make sure you by the same speed of ram you are adding to. Sometime the memory can run at a different speed than the processor bus. The motherboard sounds like it can run a CPU using any one of those bus speeds, but the memory can only go up to the 333 which is the PC2700.
There are slots cut into the circuit board that the memory is on. They don’t allow you to install buffered memory if unbuffered is required. You can find a pinout diagram on an internet site if you need it. My telling you which pin indicates buffered or unbuffered is usless without the diagram.
I would email one of the above places for help or use a site that has a program you use to select compatable memory.
Unbuffered is the norm for desktop PCs, so much so that nobody specifies “unbuffered” as opposed to “buffered” in most cases. You’ll be fine with anything that doesn’t specifically mention being buffered or registered. Only servers and high-power workstations have any need for such memory.
In your case, you won’t get anything out of PC4000 memory. It’s faster than your motherboard FSB runs normally; only overclockers would ever see a performance gain from using this stuff. Between PC3200 and PC2700, that depends on what FSB your processor has. In the worst case, however, sticking PC2700 ram in a computer in a computer that has the 400 mhz FSB will only slow it down slightly, probably too little to even notice. Still, if you can use it, you aren’t exactly saving a lot of money by getting PC2700 instead of PC3200, so get the faster stuff.
According to the relevant wikipedia page, which is backed up by its citations and other references I’ve managed to google up, the Athlon XP 2800+ has two variants, both of which have a 333 mhz fsb, meaning you should get PC2700 memory.
Faster memory just won’t help at all, but it won’t hurt (so if someone is giving away PC3200, don’t refuse it).