So given your GPU limiting resolution, anything above 2560*1440 or 2560*1080 will be pointless, unless you trade in your 5850 for something more modern but cheap and with a higher max resolution.
If you're not going to be using the screen for gaming then response times, refresh rates and g-sync/ freesync won't be required. Therefore you can concentrate on size and resolution, and if you wanted to panel type. As you've already got an IPS panel and are happy with the screen type it would probably be worth sticking with that, though modern TN panels are supposedly better than they were in terms of colour accuracy and viewing angles.
If you find text difficult to read with your current setup (which comes out at 102.46 PPI) then you might be better off using a screen with a low PPI. That could mean sticking to 1080p and going for a 24" or 27" monitor (PPI would be 91.79 and 81.59 respectively). One of these 24" IPS or VA screens may suit you:
https://www.overclockers.co.uk/benq...flicker-free-led-monitor-black-mo-103-bq.html
https://www.overclockers.co.uk/aoc-...n-led-multimedia-monitor-black-mo-036-ao.html
https://www.overclockers.co.uk/dell...im-bezel-monitor-midnight-grey-mo-072-de.html
In terms of 1080p 27" IPS/ VA screens:
https://www.overclockers.co.uk/benq...frame-multimedia-monitor-black-mo-117-bq.html
https://www.overclockers.co.uk/aoc-...s-widescreen-led-monitor-black-mo-025-ao.html
https://www.overclockers.co.uk/iiya...a-widescreen-led-monitor-black-mo-113-iy.html
If you go with a 1440p panel making sure it's at least 27" (108.79 PPI, slightly higher than your current setup). So there are a few really decent looking 25" IPS panels on the market (AOC and DELL spring to mind) but the PPI will be even higher for those screens and the jump to 25" from 21.5" may not seem that great. So for 27" 1440p panels you can either spend a little less and go VA:
https://www.overclockers.co.uk/aoc-...n-widescreen-led-monitor-black-mo-031-ao.html
https://www.overclockers.co.uk/acer...lim-bezel-flicker-free-monitor-mo-070-ac.html
or spend a little more and go IPS:
https://www.overclockers.co.uk/benq...ing-flicker-free-monitor-black-mo-110-bq.html
https://www.overclockers.co.uk/aoc-...s-widescreen-led-monitor-black-mo-018-ao.html
Finally, as you've mentioned there's the 21:9 superwide option, which at 2560*1080 and 29" would mean a slightly lower 95.81 PPI than you've got at the moment, but with much more horizontal real estate. To give you an idea of what the difference would be:
http://www.displaywars.com/21,5-inch-16x9-vs-29-inch-21x9
OcUK's stock of these types of monitors seems quite limited at the moment, and looks like the standard LG version isn't being offered anymore. As such here a couple of options:
https://www.overclockers.co.uk/asus...e-led-multimedia-monitor-black-mo-064-as.html
https://www.overclockers.co.uk/acer...r-widescreen-led-monitor-black-mo-074-ac.html
For audio editing where you'll appreciate a lot of horizontal space, I really think a 21:9 29"+ panel would be what you're looking for, and would suit movies and browsing hopefully without making you squint at text. If you go down this route, it would be worth waiting in case some newer models (possibly from LG) are released.