I've got a quad core Early 2008 Mac Pro. It came with 4GB of RAM on 4 1GB sticks.
Having recently obtained a little extra cash I decided a memory upgrade was in order as, even though Activity Monitor hasn't really shown any issues with RAM it can definitely be a bit laggy at times.

Anywho, I purchased a 8GB kit from Crucial.
The Pro has 2 memory riser cards, each with 4 slots. As per Apple rules for populating things there were 2 1GB sticks in the top riser and 2 in the bottom one. To add my 2 4GB sticks according to the rules I needed to add them to the empty two slots on the top riser.

Did that, restarted and bam, 2GB of ram rather than the 12 I was expecting. System profiler showed the top riser as completely empty.

Now, I recalled reading this article about issues someone had had with the configuration of the memory in their Mac Pro so I tried putting all the 1GB sticks on the top riser and the 4GB sticks on the bottom riser.
Things were looking up! The Mac booted and system profiler showed a full 12GB of RAM... for about 10 seconds before it restarted and showed 8GB with the top card empty again.

I pulled all the Apple ram and put 1 4GB FB-DIMM on the top riser and 1 on the bottom. System restarted and is showing 8GB of RAM, 1 on the top and 1 on the bottom.

I'm going to leave it for a bit and see if it's stable. Question is, given than I know the 1GB sticks are okay in the original configuration, is it worth having another bash to see if I can get at 12GB or should I just declare a partial victory and move on?

The only thing I every remembered was to never mix and match memory. Though I never bother with swapping memory unless I do it at purchase.

IIRC Mac pros are really picky about what memory you put in them and how you arrange it, although you'd assume Crucial would send you the right sort. Having said that I've had two lots of RAM sticks from Crucial in the last year or so that were duff in some way and caused all sorts of weird things to happen on the Macs they were installed in. I've had a couple that worked fine, to balance.

I also recall the arrangement does vary depending on exactly which model of Mac pro you're talking about. I'd download the manual from Apple support and see - some require the sticks to be filled sequentially, some require them 'interleved' i.e. half of each matched pair in the same slot in each riser. Yes, this is ruddy stupid.

if you're having trouble ID'ing exactly what version of Mac pro it is download Mactracker from the App store, that will be able to tell you exactly what model you're running it on.

It's a Mac Pro 3.1 - Early 2008 model. It says so right in system profiler. That's the model I got the RAM from Crucial for. As I said, the Crucial RAM is working fine, it just doesn't play very well with the Hynix stuff Apple shipped with the machine and this appears to be a known problem with Apple RAM and 3rd-party RAM in Mac Pro's. Still, it's a bit disappointing.