If you already have a Spyder one thing to see is if you can find an IPS display with a colour lookup table. This will allow you to calibrate your monitor, rather than butcher the output of your video card to make a monitor appear to fall in line. You actually take a quality hit when you calibrate the video lookup table rather than the monitor itself as you're no longer able to output the full 8bits per channel your card is capable of.
One monitor that fits in your budget is the NEC Multisync P241W P241W-BK, 24 Widescreen Professional Graphics Desktop Monitor - Highlights & Specifications | NEC Display Solutions . It's expensive but it'll blow away the Asus and Dell in both quality and longevity as well (I have a Dell IPS panel at work that's 3 years old and suffering from panel separation on one side, I also have an NEC Multisync 2690WUXi at home which is still perfect and pushing about 7 years now.
It looks like there's a layer of liquid inside the monitor. The right hand side has a spot which is visibly darker and spreading slowly into the screen. I'll snap a photo of it if you're interested when I get back to work.