I was just working on troubleshooting a machine someone else built with almost this exact setup. They are running some old .mov clips along with two different flash templates. I will post some updates when that machine is fixed that should give you some good minimum specifications. When I build a machine for someone, I usually recommend a quadro P1000 or better. The older K2200 is probably good enough, but I believe the K2000D doesn’t have enough of something to playback video smooth while doing graphics. There is also one machine that is running a 1660ti series GTX card that works fine. Muliti core CPU’s are also helpful.
Is this a budget machine that will only ever run this type of task for it’s entire life, or do you want space to expand? The Decklink Duo 2 would work just fine for HD work, but the Quad 2 gives a bunch of extra ports for things like clock monitors, projector management, and other features. Extra ports and processing capacity can also be used as a backup for streaming encoders or video switching if dedicated equipment fails.
I’ll post a follow up once I get that machine working properly. Let me know if you want help building a server specifically for this task.
Here is a thread to read.