First off let me clarify, you're not creating a NAS with this setup you are creating a file server. A NAS is a combination of specialized software and usually purpose built hardware (ARM SOCs are popular these days), generally managed from a webgui. In case you actually do want a NAS, FreeNAS is a good example of an opensource hardware independent NAS solution
http://www.freenas.org/
My gaol is to max out the write to this NAS for backups ( from 20GB to 100GB) comming from other PCs using a GB LAN (I have tested it and it deliver between 85MB/s to 115MB/s).
You have already a maxed out your setup. The 1Gbs Ethernet will top out at 85-120MBs (depending on, line attenuation, protocol overhead, compression and packet size). So unless you team multiple 1GBs lines together or upgrade to 10Gbs, you won't be able to read or write any faster then the 1Gbs line speed. If you're interested in nic teaming (you'll need compatible network cards and network switch), you'll be able to bump that up a bit.
http://www.aidanfinn.com/?p=12572
1 Gigabit = 125 Megabytes
Do you think that FancyCache could help if I give it 12GB of my RAM for write caching only?
No in order to fill a 12GB write buffer you would have to increase the write delay to the point of absolute absurdity (hours or days depending on your usage). As is using write delay is pretty risky.
Could it makes some conflicts as it would be used at the same time as Intel SRT?
Theoretically yes but you never know how to similar products will perform unless you test them together. Practically it would probably increase your latency, reducing the benefits of using either.
To be honest once you try to go over a network connection the bottleneck that you'll get from the 1Gbs line speed will destroy any performance gain that you'll get from using any caching software. Even a single HDD can saturate a 1Gbs link. Think about it this way, if you buy a Corvette but if you can only drive it 60 MPH what does the the extra 100 MPH that it can do matter?
I've solved this problem but it's pretty complicated, expensive and I doub't that it suit your purposes. My system contains 2 arrays 8xSSDs & 8xHDDs (around 3.5 GBs r/w). In order to move data between my storage server and my Hyper-V nodes, I had to use a 40Gbs Infiniband network (which is very $$).
I hope that answers your questions but if not feel free to post follow ups.
