Dumb ass memory question

[Crank]

okay, I warned you before you looked at this post. This is a stupid question from someone who doesn't know a lot about computers:
What's the deal with DDR memory vs. RDRAM? the DDR memory has a bigger PC# i.e. PC2100, and the RDRAM has a smaller number i.e. PC800 - but I thought that RDRAM was the fastest memory around.....
Reason I'm asking is that I'm going to buy a P4 system within the next two weeks, and now that they have MOBO's with DDR, I don't know which type to buy.
yes, I know DDR is cheaper.
Appreciate the responses and patience with this newbie question.
Thanks.

 

[AMDPwred]

If you can afford the extra cost of Rambus, I would get that.

 

[Crank]

Well, you know, it might be the difference between 512MB of memory and 1Gig of memory, but I think either size would do o.k.
Is it because the RDRAM is 'quadpumped' making it in effect PC3200 or whatever?

 

[Windogg]

DDR SDRAM was given the funny PC1600/2100/2400/2700 designation to show the available memory bandwidth. In this case, the number represents how menay megabytes per second it can theroetically transfer. IE PC2100 = 2,100MB/sec or 2.1GB/sec. This was done primarily to counter Rambus' use of the term PC800. 800 being the operating frequency of the memory. PC800 mean Rambus is running at 800Mhz (400Mhz DDR). The average comsumer might see 266Mhz (PC2100) against PC800 go, "Whoa!!! Rambus is 3x faster!!!" PC800 RDRAM actually only gets 1.6GB/sec of memory bandwidth. How is this possible?

Well Rambus is 16bit and DDR SDRAM is 64Bit. Doing the math:

800Mhz x 2 Bytes = 1600MB
266Mhz x 8 Bytes = ~2100MB

Rambus is also deeply affected by latency. The more latency, the more time the CPU must wait around.

As for DDR vs. Rambus? I personally hate Rambus the company but right now Rambus memory offers the best option in terms of performance for the P4. I bet you are saying, "Wait, if Rambus has less bandwidth and higher latency, why is it better?" Well, in the i850 chipset, Rambus is used in dual channel configuration. This means the bandwaidth from two channels are combined to effectively double the 1.6GB/sec to 3.2GB/sec. The VIA P4X266 does a good job but it still lags a little behind comparable Rambus solutions.

If you want, you could wait and get a SIS645 based system. Using 333Mhz DDR SDRAM or PC2700, this chipset give the i850 a good run for its money.

Windogg

 

[AMDPwred]

For most things you can do on your PC, the difference between 512MB and 1GB isn't noticeable at all.