Any memory configuration ok in current Mac Pros?

4 DIMMs have around 70% of the bandwidth 3 can achieve, but you likely won't notice it in real world performance. Yes you can mix capacities and do what you propose.

 

Yep.

And the additional memory gained by doing this will actually benefit the user (assuming the RAM capacity isn't more than what's needed; i.e. over-kill, which is hard to do at a reasonable cost with the '09's, and more so on the Quads).

 

The memory is configured in three channels, with an extra DIMM slot for channel 1. When running DIMMS in all four slots, memory performance is impacted. It's ideal to run just three DIMMS for full tri-channel performance. If you can afford it, it would be ideal to replace the 3x1 with 3x2GB for 6GB of tri-channel RAM.

Check out the RAM upgrades at OWC... macsales.com... they have good prices on memory upgrades and will even buy-back your old sticks I think.

 

If your current software can run triple channel, then yes. But as most applications can't come close, the additional memory would make more sense.

I'd expect this to change eventually, once software catches up. But the question is when. A few newer system generations may be out by then.

 

First of all you need to make sure you use only UDIMMs or RDIMMs. You cannot mix the two sorts. The lower capacities tend to come in UDIMM which was originally fit to your Mac Pro. So if you want to continue using them stick to UDIMM. If you consider 4 GB DIMMs you better look at RDIMMs.

Regarding the mixing of different capacities you should try to keep the three channels populated equally for maximum bandwidth. If that hurts your capacity you can deviate but you should stay as closed to balanced as possible.

Your four slots are connected to the three channels in such a way that the first two slots serve one channel each. The last two slots serve only one channel. So a good way of deviating would be to fit 2 GB modules in the first two slots and 1 GB modules into slots three and four. That way you get 6 GB, have all channels with the same capacity and get to use some of your 1 GB modules.

This is actually the way I run may octad at the moment. Octads have the same structure as quads only that they have a second memory bank which works exactly the same way.