Cable modem Ethernet into DSL router WAN port to use as a router - no DSL?

Okay, here is the problem, your required to use a custom made hardware for a specific CableCo. that is only managed by the CableCo. but you like the high speed they provide. Your support mechinism is ONLY through the CableCo. absolutely nothing in the forums will help because all management and configuration is only (normally) accessible by the CableCo. CERTIFIED TECHNICIANS (aka we got you locked up so deal with it).

Considerings your generalized statements here I would say you are running at Business Class level then simple 'Big Internet Family' many may conceive of. Your running home servers, etc. I would tell you your EASIEST path would be to discuss with the CableCo (and yes it is a higher expense) about shifting to a business class level support, because you have extremely high end needs the CableCo. consumer level doesn't support.

That said, an analysis of your configuration with multi server location, three stories of systems and wanting accessibility throughout the house I would break down the impacted factors you need to address as follows:

1) you need Quality of Service (QoS) implemented in the router AND all systems that connect (no matter the system). QoS is a levelling system to implement prioritization for LAN traffic based on the standards you implement in the table. For example the servers that do FTP could be set to Level 1, while surfing website on the Internet could be set to Level 4 so they don't interfere with moving files to / from the servers.

2) Using Gigabit Switches won't "speed up" the network, if they all connect to the CableModem that is bogged down with someone watching Funny Kitty Youttube videos. You would be better served to create Virtual LANs or direct connection paths between the servers and other 'key' equipment (aka Dad's Office computer, not Mom's kitchen PC for recipies). Normally Wireless Routers use GB on the ports anyway, but interconnection between multiple routers/switches usually needs a cross-over cable, but that is DEPENDENT on the hardware. You should sit down and plan this out more.

3) 802.11ac is on the shelf now and costs the same as 802.11n (currently common hardware) devices. 802.11ac (http://en.wikipedia.org/wiki/IEEE_802.11ac) provides not only more speed and power but most importantly MIMO (multiple IN mulitple OUT) device support. Older equipment, things take 'turns' to talk (think walkie talkie). The more devices trying to talk / listen the longer (slower) the process takes.

4) I would highly recommend puttin in a 802.11ac router in the center of the house, and add 802.11ac adapaters to all wireless devices. If the location is hefty with thick walls / floors / etc. then consider 802.11ac extenders in the farthest point (far as I know we are talking a 5000sq foot mansion!) of the signal to boost them. Add 802.11ac USB adapters and people can walk anywhere without losing signal. Run lines to the servers from this central point (300ft limit I think), then setup QoS and Virtual Lans to isolate whom talks to what device and how they won't interfere with each other. Now from that point you just created your internal LAN, now seek your CableCo support to connect that to the 'outside world', as I suggested probably a Business Class Support Level you will need to get.