Could you give us your set-up please and which software will you use?
If you're a graphical designer there are a few things you should consider before buying a display.
1. NEVER GO WITH TN (Twisted Nematic) PANELS (the hp is one). Why? image ghosting (seeing double when viewing animations), bad viewing angle's (you'll perceive a certain color differently, depending on where the pixels are located on the screen and from where you're looking toward the screen this due to= ), color shifting, fewer colors (most TN screens, if not all, can show less colors, most of the time black and white also appear respectively lighter and darker than in the more advanced P-VA and IPS panels).
2. There are two other groups of techniques to arrange the crystal matrix of the pixels. Namely P-VA and IPS. I'm not sure why but mostly IPS is considered superior in comparison to P-VA (as is the price-tag ...), although most of the high-end displays you can buy nowadays use the IPS technique (all mac's included - except maybe the first gen mba). What are the advantages of these techniques over TN? well just reverse all the disadvantages I mentioned for TN

. Also: I'v never seen a TN-panel with a resolution higher than 1920x1080.
3. Never believe the salesman. If you see a display that costs half of what you expected it to cost, there might be some unwelcome surprises after the unboxing. Therefore ALWAYS look for reviews on the net. There are plenty of sites whit this kind of information. Just google "<name of the screen> review".
4. Check out your system and compare it to the recommended specs of the software you'll be using. If your system only meets the minimum requirements, don't expect it to be capable of running a dual screen set-up or a 2560x1440 screen.
Hope this helps
Personally: I'd go with the Dell 2711. It's about 200 bucks over your budget but those are really worth it in my opinion. Great screen, for a great price.