No it just means that it's the eighth revision of the iMac line. Kind of like with the iPhone: Original iPhone (1,1) iPhone 3G (1,2) iPhone 3G S (2,1)
EDIT: Sorry didn't even read the entire question like an idiot lol, the iMac 8,1 is the first version of the aluminum models that was introduced in August 2007 i believe, it uses 667 MHz DDR2 RAM. I know that one for a fact considering that's what I use. Plus it can take up to 4GB of RAM so you should be able to buy it for a relatively low price.