I've been doing a lot of GPU programming these last few years. Now, FPGAs are being used to accelerate tasks (algo trading, etc, etc).
Recommendations on how to get started with FPGA development? What cards are good? Do they make external cards to plug into a laptop like eGPUs?
Popular SDKs?
2) The two major FPGA manufacturers are Intel and Xilinx. Each has their own tools. The tools are very different. You should pick one (flip a coin) and stick with it, at least starting out.
3) FPGA manufacturers typically have a free tier of their tools, and a paid tier. The paid tier is usually thousands of dollars. You'll only need the paid tier if you're using a high-end FPGA (those are usually also very expensive - thousands of dollars for an evaluation board).
4) Buy a popular FPGA board in the $100-$1000 range. I recommend something from Terasic (Intel FPGA boards) or Digilent (Xilinx FPGA boards).
5) Accept that you won't have a high-speed connection to a PC. Inexpensive FPGA boards normally only have gigabit ethernet, at best. You can get a PCIe FPGA board, or connect a USB 3.0 PHY via FMC connector, but you're looking at more money invested, and you'd have to learn a lot more about the inner workings of the FPGA.
6) The tools you're looking for are Intel Quartus II HLS, or Xilinx Vivado HLS.
7) Look into OpenCL for Intel and Xilinx FPGAs. This is the ultimate goal you're trying to attain - getting arbitrary algorithms optimized for FPGA fabric. Good luck!