I got RMA replacements for 2 kits of Geil DDR550 and in my DFI NF4 Ultra-D using a Venice 3000+ and with all 4 dimms it does 275 FSB with the memory at 1:1 no problem (vdimm is only 2.6v):
The default when BIOS options are set to auto is DDR333!
It does depend on the motherboard and CPU but I'm sure with a decent board you should have no problems with your Opteron 185. The main concern would be problems related to the other dimms SPD settings being different but you would likely be able to fix such issues by modifying the SPD so they match.