Those things mean nothing, you're dealing with morons. Look, there are TWO versions of the P4 1.8GHz CPU, the Williamette and the Northwood. The Williamette is the older, far slower version. It has 256k cache and runs hot, using a .18 micron die process. The Northwood runs cooler on it's .13 micron die process, has 512k cache, and is highly overclockable.
Some people call the Northwood 1.8GHz the 1.8A. Others call it a Northwood, or a P4 512k. Once you know the specs of the two processors, it's easy to figure out. It's just that dealers have different ways of specifying them.
The G means nothing. It's probably an abreviation for GHz. 1.8G and 1.8AG are therefor not meaningfull names, unless the same dealer sells both and you know he's trying to separate the 1.8 Williamette from the 1.8 Northwood with those descriptions.
Anyway, my suggestion is to read the description of the CPU, instead of going by titles that mean little or nothing.
<font color=blue>You're posting in a forum with class. It may be third class, but it's still class!</font color=blue>