SDRAM = Synchronous Dynamic Random Access Memory. I think you are not understanding that RAM, that isn't Rambus, is SDRAM. SDR (Single Data Rate) is the old style RAM, where DDR (Double Data Rate) is what you are using now, and need, but they are both still SDRAM; even DDR2 is still SDRAM.
You can also get faster RAM that what you are using. If it were me, I would get PC3200. It is no more expensive, and will be easier to use down the road, even though it is being phased out by DDR2. Just remember the PC3200 means the RAM is rated to run at 200MHz, but can obviously run slower than its rated speed, so running at 133MHz like your current RAM is not a problem. In fact, I have an old system at home with an XP1700+ and KT266A chips that is running with 1GB of PC3200 RAM.
As noted already, registered and buffered RAM are the same thing. Buffering is primarily seen in servers where very large amounts of RAM are used. The buffering delays the read/write signal by one clock cycle because the buffering electronics take the load off the memory controller, but it keeps the signal clean and strong to prevent signal loss and ground bouncing. I would not get registered RAM.
I would get PC3200 DDR-SDRAM, unbuffered, non-ECC.