In IDA I have seen the parameter listing inside functions contain any of the following...

arg_0
arg_C
var_1
var_2
var_3

I'm assuming there is some sort of logic to the naming convention, but I haven't figured it out.

Also, what register do *return* values for functions get placed in? For instance a program that looks like the following...

push [ebp+var_1]
push [ebp+var_2]
call function_12345

...where the function would take those two parameters, manipulate them and return execution to the calling segment.