Might want to look on web for computer ergonomics.
Should give you position of arms and head for most restful position.
If you use a pad , keyboard, and a mouse make sure you have enough room for them at the correct height.
I find those dedicated computer desks with a pull out surface for the keyboard and mouse impossible to use because the keyboard is too low, but I think they are OK if you can touch-type. You need room for two monitors, (even if you only use one now) so that probably means a flat desk top, and not one with built in shelves etc. at the back Just lots of desktop space really.
Using a keyboard and graphics tablet can lead to problems. The keyboard can be too far away to use comfortable with the tablet in front of it. I used to feel OK with a little Graphire to the right of the keyboard just beyond the mouse, but a mid size Intuos really needs to be in front of you. I overcame this by making two wooden wedges that span the tablet, and support the keyboard at a bit of an angle. This lets me slide the keyboard towards me over the tablet when not using Photoshop, and back towards the monitor when using Photoshop. The angle raises the back of the keyboard making it MUCH easier to use.
Note the little black super slippery mouse mat. It made heaps of difference to my Mine Sweeper scores.
Here's what works for me... I'm a software engineer and use my desktop computer virtually constantly, for Photoshop and many other things.
I have a 25" high office table on which I use a desktop computer. I sit at the left end of it, and had a custom back built that goes up the wall and provides a bunch of shelf space.
On top of the computer I have a board that makes it a bit wider, and on top of that I have a pair of 20" monitors side by side. The height of the center of the monitors is exactly at eye level, and I can just touch the monitor screen with my fingertips if I extend my hand. There's just enough space in front of the computer on the desk for my keyboard. Interestingly, I felt a reduction in physical stress level when I moved from one monitor to two, since I think the head motion required to turn to see each keeps me more loose.
I have a clipboard that I push under the right-edge of the keyboard and which extends out from the desk a little toward me - I use this as my "mouse pad". I found this slight additional extension to be very important for me to maintain wrist comfort, since I can rest my palm easily on the board without having to extend my arm forward.
The desk is 27" deep, which allows the full-sized desktop computer and keyboard to fit with just enough room in the back for the cables. It's all open underneath, all the way to the wall it's against, so my legs have a lot of room.
My office chair I got at Office Max, and I chose it by sitting in all of them there. It was one of the few that made my back feel better when I sat in it. This chair is good because it allows me to adjust the height, and I like to set it so the arm rests are at the same height as my keyboard and mouse. It's a typical chair, with wheels, and it swivels. It's also slightly springy.
After a lifetime of computer work, I've settled on this combination. II can work here for an unlimited time without fatigue.
Apologies for the mess.
I didn't think folk used non-tower cases nowadays. Cooling is not so good, and not much room for extra drives and a decent graphics card.
I converted it, so the DVD drive is horizontal. It puts my monitors up at just the right height.
Cooling is fine, I have 3 TB of internal storage in 3 drives, and and my graphics card has plenty of room. It's a full-sized Dell Precision workstation chassis.
Like Noel, I bought both my desk and chair after sitting in front of, and on, them at the store to make sure they were comfortable... I really think that's the only way to do it; try it and see what works for you. We all have different needs and requirements, and for the most part will probably strike a balance somewhere between ergonomics and aesthetics.
For myself I have a single 24" monitor, a keyboard, trackball, and medium Wacom tablet. The keyboard and trackball are on a sliding keyboard shelf beneath the table, I can touch type pretty well, with the trackball on the right as I'm right-handed, and the tablet sits on the right front edge of the table; my full tower case and back-up drives sit on a separate low "parsons table" to the right of the computer desk.
I will say that with us all rightly considering the ergonomics of our set-ups, why trackballs have not been more widely adopted really surprises me! I've been using trackballs for over 10yrs now and really hate it when I have to work on someone's machine with a mouse; all that pushing and shoving trying to move the cursor around the screen... YUCK! LOL!
I use the Logitech Trackman, the one where the ball sits under your thumb; there are two main buttons on top plus a wheel, which can also be programmed as a third button. You place it in position so your hand naturally falls on it, no having to pull or push it into position on a mat before you can start using it, and I can fully traverse my screen either horizontally or vertically with one smooth movement of my thumb... no RSI for me!!!
All the best