Waco, I don't suppose you'll be at the BBQ. If love to pick your brain.
So, with L2arc, will I need another VDEV? if so, will it need to be redundant? I have 16 GB on a single stick at the moment, so I will likely be adding another for dual channel 32.
I wish I could be there, but unfortunately not going to happen this year (again).
The L2ARC does not require redundancy. It is technically another VDEV, but it's a special type. If it fails, it just faults the VDEV for caching and you return to the standard behavior without it (along with some nasty emails if you have zed set up). The ram usage is dependent upon how much data ends up in the L2ARC (number of records, size of records, etc). There's a formula to figure it out that I don't have on the top of my head, but if you're using 1 MB records (likely on a data pool) the limit is pretty darn high before it matters. A couple hundred GB of L2ARC will happily fit in a few GB of RAM.
In general, though, L2ARC isn't super useful for most. More RAM is almost always a better purchase unless you have very specific workloads that blow out RAM consistently. If you're just loading occasional (older) games as your performance sensitive workload, I bet you'd see little benefit. I used to run an L2ARC and even running VMs directly from my NAS I rarely saw hits in the L2ARC.
EDIT: If you really want to ensure a minimum level of performance, though, you could always set up your L2ARC to only cache your dataset that's sensitive for performance and disable it for all media. With enough data accessed consistently (larger than RAM) it could help significantly. Testing is the only way to find out, though.
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NAS: 1950X | Designare EX | 32 GB ECC | 7x8 TB RAIDZ2 | 8x2 TB RAID10 | FreeNAS | ZFS | LSI SAS