Yeah, the 1.8 requires PC3200, so either
1) it just won't boot because the Mac will detect a non-compliant SPD
2) the RAM iwould be run at higher than rated speed (increased chance of error and failure) or
3) if it runs, the entire machine would be dragged down to 333 MHZ memory speed.
Not worth the risk IMO
The only G5 that supports PC2700 is the single 1.6