I think "good computer screen" and "under 300-400$" may be mutually exclusive.
I use an NEC PA271W which is 27" and cost about $1600. It's wonderful and works great for picture editing**. But to be honest, I could have been just as happy with the 24" version of the same monitor. The difference in size between the 24" and 27" is smaller than you might think. The 24" is about $1300 but sometimes goes on sale for $1200.
The way I see it, a monitor lasts about four years or so. That means a $1300 monitor costs less than $1 /day. A good monitor is as important to photo editing as good tires are important to a car.
The more expensive NEC monitors come with built-in colour calibration (called "Spectraview"). It's software plus a colorimeter tuned for the monitor.
NEC makes a variety of displays
(the site has US prices) and they are sold in Canada. Since most photo software has a large number of tool palettes and other user interface stuff, a wider screen comes in handy. It's also nice if the monitor has built-in USB ports. I use them for headphones, iphone charger and card reader.
**The only problem with high resolution monitors is that most, if not all, photo software is not properly designed for these types of displays. With almost all photo software, the text, tool icons, sliders, etc., are tiny in size. Plus, many photo software companies follow the silly trend of using grey text on grey backgrounds which is tough to read. (A blog post
on my site about this.)