That is a pretty good budget for a good quality monitor using a S-PVA / P-MVA or IPS panel which basically starts at $550. Less expensive 24" monitors use TN panels which generally have faster response times so many gamers tend to stick to those types of monitors.
Other than the price difference there color accuracy and general image quality issues. TN panels tends to have:
1. Color banding issues when there are subtle colors changes. This is mainly due to the fact that these panel can only produce 256k true colors, through dithering it can blend those colors up to 16m.
2. Viewing angles are more limited due to the limitation of the tech itself so colors looks fade and shifts shades the more off center you are when viewing the monitor. Even when looking directly at the monitor average people can notice that the top of the monitor looks darker than the bottom of the monitor.
3. Backlight bleeding refers to the florescent light behind the panel "bleeding" through even though the screen is displaying black. That means images or colors that you see are a little brighter than they should be. This mainly happens near the edges of the screen, but can also affect the entire panel itself. From my experience TN panel monitor seems to exhibit this "defect" more often and more severe than PVA / MVA / IPS panels.
The following is a good resource to understand LCD panels, they also have some reviews:
I own 3 LCD monitors which are based off of TN, S-PVA and H-IPS panels.
I generally do not notice any ghosting on my NEC 2690WUXi (H-IPS panel) which is rated at 6ms response times or my Asus VK246H (TN Panel) which is rated at 2s response times. The NEC is my primary monitor, the Asus will only be used a few hours a week so something of adequate color quality is good enough.
However, when it comes to movies I definitely prefer my NEC because it can actually produce 16.7m colors. TN panels can only produce 256k colors, thru dithering up to 16.7m colors can created (or so they claim). This leads to color banding and image artifacts. It may not be noticeable to most people, but I can see these issues especially when watch the Dark Knight DVD.
My Planar PX191 is the first LCD monitor I bought back in 2002. Watching movies is pretty good since it can actually produce 16.7m colors just like my NEC. Movies on the NEC looks better though 'cause it has extra electronics for better color accuracy and color uniformity. Gaming is another issue since it probably has a GTG response time of 16ms (just a guess). BTW response times were used back which was 25ms for this particular monitor.
People now look at input lag which is the time delay between say moving the mouse and the movement being displayed on the screen. Hardcore gamers will probably notice the lag in first person shooters, but average gamers probably won't notice it. My NEC 2690WUXi has been tested to have an average input lag of 32ms or about 2 frames. That is not noticeable at all to me.
If my Planar PX191 died at this very moment, then I would consider HP LP2475w since it uses a H-IPS panel (like you mentioned) and has an average input lag of 25ms. Of course if price was not a factor, then I would buy another NEC 2690WUXi (I guess I could start saving now for it).
Many people like the Dell 2408WFP, but the main complaint is that the first version (revision A00) had an average input lag of 50ms or about 3 frames. The newer A01 is said to have lower input lag, but I've so far have seen no test with that data. Maybe it will be better than the HP, but probably not.