is 640MB of good ol' pc-333 sdram comprable to 256MB of DDR in terms of performance??

no, DDR has double the bandwith, if you're playing games or running CAD, DDR will be faster/react quicker, and it's pc-133

In theory but your don't notice double the bandwidth in real use. More like 25% more bandwidth. The only real difference when using DDR RAM is the quicker boot process that will occur compared to PC133.

C'mon guys, it's a loaded question. The answer is yes and no!!! It is true that DDR SDRAM can process twice the information as a regular SDRAM and that's not in theory, and they accomplish it at the same frequency, either 100MHZ for PC100 and PC1600(PC200) and 133Mhz for PC133 and PC2100(PC266). PC133 can handle information at 1066 MB/sec while double data rate(DDR) can process 2100 MB/sec at same frequency since it handles information at both up and down of the cycle. As you can see, the 2100 marking stands for MB/sec and not Mhz. PC100 can handle 800MB/sec while it's counterpart, PC1600 DDR will handle 1600 MB/sec, both at 100MHz. Because of this disparity, my first answer to you question is no. DDR IS FASTER!! If I will look deeper into your question, I may have a second answer. During normal operation, a similar system that utilize DDR will always be faster. However, when you start multi-tasking and when using memory hungry programs, the DDR will lose it's edge since your system will start using swap file after 256MB. I don't care how fast your HD is, ATA/100 @7200RPM is way too slow as compared to RAM. Your other system will not using swap until after 640MB. Although 256MB is not less neither, on certain conditions, that extra 384MB will drag your system down when swapped to a hard drive. Therefor my second answer to your question is YES, they are comparable!! You can also ask the same question, "is a 120MHZ 486 with 64MB RAM comparable to a Pentium 75 with 8MP RAM?" again the answer is yes and no. If your original question is a legitimate one and not a typo, 333 Vs 133, then I will assume that you are talking about Rambus(RDRAM) and not SDRAM. These are most commonly used in game consoles. However, A PC800 RDRAM running at 800MHZ transmits data at only 16 bit increment, while DDR and SDRAM runs at 64 bit. Again the possible third answer to your question is No.

the judge, thanks for your very informative response i currently have 640MB of pc-133 (i apologize for the typo) running with an Athlon 2200xp+ and i utilize the system for gaming...i was considering removing my large amount of pc-133 sdram and putting 256MB of DDR ram into it... i hesitate to do this when you consider im swapping 640MB for only 256MB even though it is DDR i have the latency on DDR can actually cause it to run slower than sd-ram and also true "double rates" are not shown when using it. i really cannot make the decision as to go with my large amount of sdram, or shrink the amount of memory i have to utilize DDR

i have (read) the latency on DDR....

Don't worry too much about the Latency, CL or CAS Latency. CAS Latency or Column Address Strobe Latency is the time in clock cycles between the time the processor requests data from the RAM chip to the time when the data arrives at the data bus. CL2 (Cas Latency 2) is a little bit faster than CL3 (Cas Latency 3) because during one step in the column accessing process you only have to wait two clock cycles instead of three clock cycles as with CL3. The overall performance difference is minor. There is less than 1 percent of performance difference between a CL2 and CL3 part.