>is it possible that 4000/90 is really twice faster than 4000/60 ??

Yes, ist is - at least under OpenVMS. Besides the faster processor it 
has a better memory bus and network interface. I don't know about SCSI, 
but under NetBSD that speed is not comparable anyway.

>THen I Also read that on 4000/60 can be mounted up to 104MB RAM.
>Where I Can find , I mean buy the SIMM modules for 4000/60 ??

The machines use 70ns 80pin SIMMs, which are hard to get. You can try to 
get them on eBay or something like that. The VAXstation 3100 M76 uses 
similar SIMMs but at 100ns, which don't work with the 4000-series.

Bye,
Matthias