I do have got a RBM A1200 tower working with ONLY a 1220/4 68020 turboboard with ONLY 4 Mb Fast Ram and OS 3.9, so if you say 6 Mb is needed then I assume you mean 2+4 Mb
>4 Gb harddrives ... there is a patch on Aminet to use bigger harddrives, even on OS3.0 with kickstart 3.0 ...
Turboboards ... take a look
at the Belgian Amiga Club site where I often keep track of several Amiga products being sold on Ebay, including turboboards, so it will give you a somewhat objective idea about what things may cost ...
