I'm just starting my career in software development, and I'd like to get a great chair, keyboard, mouse, monitor, etc
Odds are that if this your first gig as a professional software engineer (developer) you are fairly junior and working for a real company. If that's the case, you will get whatever they give you, augmented by whatever you can steal (aka - repurpose gear that you find unattended and unspoken for.) Be prepared - they will probably try to stick you with whatever the last guy left behind at the desk you inherit, and it will be crap.
a comfortable, ergonomic, efficient work environment
Since the ergonomics aspect of your question has been covered sufficiently above, I am going to suggest the the best productivity enhancer I have found yet : multiple high density monitors (1920x1080's, or 1920x1200's if you can get them). There is no substitute for screen real estate, esp when running code in the debugger while driving the application on a different screen.. You are going to find the most effective force multiplier comes with the second monitor attached to your existing machine, with a close follow-up by adding a second computer with two more monitors connected to the first one via Synergy so you control both via one mouse / keyboard combo. Simply move the mouse from one system to the other system and you are controlling the other system, with the ability to cut-n-paste between them. The second machine can be something very weak, you will relegate it to browsing the web (Googling API calls, etc, running the application you are working on (ie, end user testing), etc) - doesn't have to be a powerful workstation.
Note - you are going to encounter a LOT of push-back and resistance from whoever is buying when you order four full size monitors and two machines. Back in the day monitors cost $700 apiece and when someone who has been doing this more than a few years (ie your manager, other more experienced developers) see more than one they see 'crazy expensive', when the reality is that if you catch them on the cheap you can get four full size LCD's for less than a single monitor used to cost. Start with at least two, though, because the performance gains really are all you'd imagine and probably more.