Precision T7400 memory upgrades

So I've got 4 of these things, we want to add 8GB to each one.  One pair has X5450 3GHz 1333 MHz FSB CPU and the other has X5482 3.2 GHz 1600 MHz FSB CPUs.  Manual says to install matching quads for best performance.

I considered getting the upgrades from Dell directly but these 2x2GB kits seem to be perpetually sold out?  Also couldn't find it on the Canadian site (and don't want to pay duty), only this crazy expensive kit, 8x$280 ouch. So gave up on the Dell site (IT guy suggested going to a place like Newegg anyways) but I'm a bit nervous about compatibility.

I was looking at buying 8 of these 4GB ddr2-800 sticks, putting a quad of them in each of the X5482 machines and then shuffling their 4x2GB ddr2-800 into the X5450 machines. The appeal of this is that it is the cheapest option at less than $1100 before tax and shipping.

However looking at the Crucial site, they have this ominous text about the T7400: "800MHz FBDIMMs are for 1600MHz FSB CPUs only." (Looking at the config pages, the X5450 machines did come ddr2-667 and the X5482 ones with ddr2-800)

So plan B if I need actually get the matching speeds is to get two 2x2GB kits for each machine. These ones for the ddr2-667 machines and these ones for the ddr2-800 machines. Cost here is $1234 before tax and shipping.

Any confirmation, help or suggestions would be appreciated.  My two main concerns are whether those sticks should work in general (they are ECC and fully buffered), and also whether the ddr2-800 sticks would work in the 1333 MHz FSB machines despite Crucial's warning.  Thanks.