Hi Lars,
the most important advantage of a IPS panel over a TN panel is in my opinion that the colors and contrast will not alter with different viewing angles. Depending on the space of your desk and the size of the screen this could be a great benefit. I don't want to miss it. The downside is there can be some sparkling effects and the slower response time of the panel. It doesn't bothering me at no time, especially in terms of image editing.
Have a look here:
http://www.prad.de/en/monitore/review/2012/review-eizo-ev2436wfs-bk-part7.html#Viewing
Yes, professional displays (like the Eizo ColorGraphics or NEC MultiSync PA Series) have a better stability of the colors/gamut over time. The advertising this something similar like "Backlight Ageing Correction". Nevertheless, you will have to recalibrate these displays from time to time as well, but the difference shouldn't be so much. I don't know whats about the FlexScan Series, but I guess it should be much better than your old 19 inch DELL display.
Color calibration/profiling and color management are quite complex topics. I'm not an expert on it, but I gained some knowledge over time. I think it would be to much to explain everything here, I will try to keep it simple:
First, there's a difference between calibration and profiling. Usually you will do it as one task, but for real there are two steps.
1. Calibration brings your monitor to certain specs, like gamma, brightness level, white point etc. If your monitor is not hardware calibrateable you have to do manually and a cheap bad monitor have less and unprecise controls to modify your output. Your colorimeter supports you in the process, but usually it's time consuming and inaccurate. Better monitors have more controls like 6-axis gain or even more precise tools to achieve a good calibration by hand. With hardware calibratable displays your monitor is communicating to the colorimeter directly. This process is more or less done automatically and very accurately.
2. The profiling comes after the calibration and measure the differences from standard spec to your actual output. The accurate the calibration and the color reproduction of your monitor is, the less changes has to be done in the profile. Very good monitors have a programmable LUT, that means the monitor process NEARLY everything by it self. This can be very accurate and reproduces colors or color space transformations very precisely. Otherwise, with non hardware calibrateable monitors, the graphics card of your computer is responsible for the color correction. But it is not that precise and the more correction has to be done the more information can be lost (resulting tonal breaks or right colors in one spectrum and false colors in another).
A monitor with bad color reproduction is still bad even when it software calibrated. Sure you will maybe improve some color reproduction to have some colors looking right, but you will also loose information. That means that the color shifts after profiling and corrected by the graphics card can cause tonal breaks (as explained a little earlier). Otherwise the total amount of the displayable colors can be very import. The calibration/profiling process cannot conjure up colors that are not displayable by your monitor. That's the reason why the monitor vendors advertising a color space match for sRGB or Adobe RGB etc.
A good medium range monitor has usually more representable colors and more control over gamut, brightness etc.. After calibration/profiling you will only do some small corrections, the overall color performance would benefit from it.
A hardware calibrated and professional monitor can be very precise because the monitor has his own processor for transforming the colors and you will avoid tonal breaks as much as possible. A lot of new monitors will have 10 bit (via Displayport connectors) color output, that means, that you will able to reproduce about a billion colors. But you will need a professional graphics card and software which can support 10 bit color output. Possible cards are Nvidia Quadro or ATI FireGL cards. The only software supporting 10 bit output is as far as I know Photoshop. These can be great benefits in my opinion, even gradients will be very smooth and you will not see tonal breaks so far. Of course you have shot in RAW and at least 16 bit images during post production.
When you are producing images mostly for screen output and you don't depend on a 100% color reproducible workflow, I think you don't need a very expensive monitor. Even when you are printing from time to time for personal purposes, a small amount of inaccuracy is tolerable. All the precise calibration and color space emulation is nice but I think it is more reasonable for printing or proofing, even in accurate color reproducible workflows.
So a good medium range display would do it.
I hope this will help you a little. Maybe I will find a good web reference about this topic, if you are interested?
Good luck,
Peter