Is it possible to have 3GB RAM?

Yeah you could. There are 8 slots available but RAM can only be installed in pairs. If your ram is 4x512 then you only have 4 slots available and could buy 2x512 to make it 3gb. You mac can have a total of 16gb of ram.

Yes, 3GB is pissble with 6 x 512MB DIMMs. I actually have this configuration now.

I just bought the basic 1GB (2 x 512K) with my Mac Pro, then added 2GB (2 x 1GB) from Crucial for 3 GB total.

Probably for no other reason that simplicity. Given the use of 512MB, 1GB and 2GB FB-DIMMs and 8 slots there are a substantial number of combinations available.

6 x 512MB is not one of them. While not as nice as 2 x 1GB + 2 x 512MB it does perform better than 4 x 512MB. The difference come in when you have the choice of using larger DIMMs or not. For instance, a 4 x 1GB configuration will perform better than 8 x 512MB configuration. Having said that there will be very few that would be able to notice the difference.

I thought that it was better to keep the sizes the same. So as long as the pairs are matched it doesn't matter?

For Example

2 - 512MB, 2 - 1GB & 2 - 2 GB

or would just expanding with 2GB modules be best

Power Book - Mac Pro Coming Mac OS X (10.4.8)

Yes, you need to install in pair although there are 4 pairs to fill as you choose. There are often multiple configurations with the same RAM total and the performance of each will not be the same.

There are plenty of threads that discuss this in depth.

so, 6 x 512's would not be slower than 4x512's

i would think it would introduce some latency.

that is the only thing stoping me from buying 2 more 512'd

OK… would the RAM in slots 3/4 (either riser) perform slower than the RAM in slots 1/2 due to latency/FB-DIMM spec… yes. The thing is, if you need more than 2GB RAM, would that extra 1GB not operate faster than virtual memory on your hard disk?! While it's true it's not perfectly optimal it's certainly far better than the alternative. In any case, only in rare case would anyone actually notice the latency difference.

so you would go with buying 2 more 512's.

I have 4x512 right now. I want to upgrade. But buying four 1 gigs sticks is expensive, plus then i would have four 512's to sell.

But i could just buy 2 more 512's, and add them in for cheap.

your saying the performance increase would not be cancelled out by latency, and it would be worth it?

i hope that is what your saying. if it is I will buy two more 512's. let me know if you could please. thanks very much,

If you look at the 3rd post in this thread you'll see that I stated that I have this configuration now. I don't know what more you want anyone to say.

But, would it be better to have four one gigabytes chips vs. 6 512's, or not worth the extra cost.

If i bought the 4 1's. i could also use my 4 512's for 6 gigs total. i just want know what is the magic number. i heard 4 chips was the best to have. any more and you are causing latency. but like you said. surly this latency is not slower than virtual memory. are you sure of that though?

I think Ned said it all.

4 sticks populating slots 1 & 2 on both Risers, sets the memory usage to Quad channel. Installing more sticks of memory over 4 will still allow for Quad channel, however does casue some latency.

The latency caused is in nano seconds and so should never be a problem or noticed.

These things run so fast that we end users read latency issue and we start to panic - 'I don't want a latency issue'. 'why me? why me?'.

The speeds they are talking about are almost impossible for anyone to register. Have you got a nano second hand on your watch?

With a maximum of 16Gb RAM, the combinations of configurations are many and varied. Rule of thumb, use bigger sticks of memory in slots 1 & 2 on Riser A first, then slots 1 & 2 on Riser B, then populate slots 3 & 4 on Riser A then 3 & 4 on Riser B (e.g 1Gb paired sticks in slots 1 & 2 on A, 512Mb in slots 1 & 2 on B or 1Gb sticks in slots 1 & 2 on A & B, 512Mb's in slots 3 & 4 on A - and so on)