I'm guessing that 7.5 means your OS and therefore your machine sounds as if it is pre powermac(G3). Macs of that ilk will be difficult to upgrade not least because of the lack of available parts. Their HDD tend to be SCSI. The memory is the same as PC but you need to know what is already in use before you buy more.
The thing with macs is they were more or less redesigned from the ground up with each new model which make interchanging parts very difficult as opposed to PC.
Personally I'd consider upgrading to a newer machine. I've got 3 G3's in my shed waiting for attention but sourcing parts is the tricky bit....see what I mean? Treat yourself!