ram runs at a ratio off the fsb or htt speed, and DDR266/PC2100 should work BUT it would absolutly kill performance cause its around half the speed its designed for! buy new DDR400 ram with your upgrade!!!.
Apache_Lives is right. But if you really have no budget left at all you can still run your rig with the crappy ram until you find money for decent one. The fastest pc i can get my hands on is running PC133. 120MB of it. 'cos the rest of it(8MB) goes for the crappy integrated Intel videocontroller. Nice huh?
Can you? Yes. (At least I think so.) Do you want to? Probably not. It'd be like buying a new car without tires and then driving it on the rims. Technically you could do it, but you're not going to be going very fast.
All you should read this ...about AMD Athlon/Opteron processors ...
The memory connections are direct, glue-less, 184-pin DDR SDRAM channels directly connected to the chip. There is one DDR channel in Socket-754 and two DDR channels in Socket-939/940 (noting that 754+184=938, there are two, true, full, non-multiplexed DDR buses in the case of Socket-939/940). There are several advantages to this. One is the fact that memory speed no longer limits the performance of any other interconnect. In other words, the memory clock is its own, independent divider of the local CPU clock -- separate from all others, including I/O. Using DDR266/PC2100 (2.1 GBps/channel) instead of DDR400/PC3200 (3.2 GBps/channel) makes no impact on the CPU speed or HyperTransport clocks. And, it gets better.