As subject line above read, looking at some units which are rated at GH/s and would like to know what they work out in the more popular MHash/sec.
All GPUs are under 1,000 Mhash/s (for bitcoin mining) so the unit for describing their performance is in Mhash/s.
There are FPGA units which perform at better than 1,000 Mhash/s so their performance might be rated in terms of Ghash/s. 1 Ghash/s = 1,000 Mhash/s.
ASICs also are above 1,000 Mhash/s (well above) so they too are rated in terms of Ghash/s as well.
Now this is just for Bitcoin mining. Other alternative crypto currencies might use a different algorithm and thus a card that performs a certain level for Bitcoin mining can have a completely different rating for the alt chain mining. Litecoin, for instance, uses the Scrypt algorithm. So an AMD 5970 that might do 700 Mhash/s when mining Bitcoin might do under
100 10 [Edited] Mhash when mining Litecoin. The difference would be on a per-model basis versus there being a simple X:Y ratio for comparing how one model might perform versus another.