If you want to save yourself a LOT of time and effort.
Open the 660 box you have and remove the PCIE video card. It's no different than removing a PCIE card from a desktop PC. Then remove your existing brainbox, pop the cover and plug it in.
If the 660 has 4GB of RAM and your existing box only has 2GB, swap the RAM from the 660 as well. And while you're at it, change all the batteries unless you're sure they're fresh. Two CR2450s in the PCI card, and a CR2032 in the motherboard. Will save you a lot of time if the batteries aren't known to be fairly new.
Put your brainbox back in as you have the matching power connectors and voila. Exactly the same as a 660.
But you don't connect your displays to where you used to. You connect both your displays to the dual DVI card now. Onboard video will be disabled when the PCIE card is present. So you'll need a DVI male to VGA female adapter to connect the existing VGA cable for your lower screen to as the lower screen must be connected using a DVI > VGA adapter > your current lower display cable or things won't work right. Connect your top display with a DVI > DVI cable which it should have there already.
If you have the parts on hand, about a 30 min job and you're good to go. You can get the batteries and adapter from Amazon.
DVI>VGA adapter
https://www.amazon.com/gp/product/B009JXH8TU/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1PCI card Batteries (2)
https://www.amazon.com/gp/product/B00O35ULSY/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1Motherboard Bios battery
https://www.amazon.com/gp/product/B00D8P5T0U/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1 (many other things use these batteries so you'll have some around)