How possible is it so that they work if i switch the hard disks of these two Powermacs? Should there be any conflict if i take the 1.8Ghz HD to place it on the DualCoreXeon and vice versa?
Technically the answer is no, because the Power Mac has a drive with a partition scheme called Apple Partition Map, while the Intel Mac Pro uses GUID Partition Table.
It's easy to reformat a drive and change the partition scheme with Disk Utility, but of course you'd lose all your data.
I think there *is* an exception, is I remember correctly: apparently a Mac Pro will boot up just fine if you pop an internal hard disk from a PowerPC Mac partitioned with Apple Partition Map, so long as OS X has already been installed on the drive. Intel Macs simply would install OS X onto an Apple Partition Map formatted drive.
Not sure if that OS would then boot up or work properly if you tried to update to Snow Leopard on the Intel Mac. Actually, I'm pretty sure it wouldn't work, because the SL/Lion/etc installer would refuse to install because the drive is still APM.
You can put G5 HDD in MP and it should work and boot OK, unless you've done some Monolingual or Xslimmer "magic".
Vice versa is possible, but not for booting purposes. G5 won't boot off of GUID partition.