BrownBaron, I think you should start with your allotted budget. I find that knowing what my spending limits are is the first step when I'm contemplating an upgrade. Most people probably take that approach. If your budget is limited, it often pares things down and makes your choices obvious.
Next, assuming that your budget allows you to buy any of the items discussed above, I would use this rule of thumb: Performance... then Immersion.
Leaving aside performance gains that you might get from spending your budget on a new graphics or sound card (for the sake of argument, let's say your hardware is in good shape and you're okay on both of these essentials for the time being), IMO, you should then look to what
in-game performance enhancements your budget will buy you
before buying any
in-game immersion enhancements with that limited budget.
For me, the order then becomes: High quality controllers (brand of choice, but for me, that means a full HOTAS rig with rudder pedals)--->high quality headphones/mic (brand of choice)--->TrackIR or FreeTrack--->Large monitor or 3 monitors-->full simpit (truly only for completely addicted).
Some of you would say some of this is unnecessary, frivolous, or my order is wrong (possibly true). My point here is that, with all the gizmos and gadgets we can apply to this addiction,
figuring out an order of progressive investment in what you implement is a good way to get started. It puts your highest gains in your earliest purchases, and makes it easier to budget for the later items.
Bottom line for me: Just use common sense and start with what will immediately impact how you perform in the game (for me, that's the controllers), then move outward toward the immersive stuff like a big monitor or multiple screens.
Good luck and have fun.
FWIW,
