A couple things:
1) A 27" at 1080p is too big, running the same number of pixels it will start to lose some sharpness at normal viewing distance. I don't recommend it.
2) For gaming 120hz is far more important than anything else. A 60hz monitor will always have at least 16ms of input lag because that is the amount of time between frame refreshes, a 120hz monitor halves that to a mostly unnoticable 8ms. That also means much better smoothness and responsiveness, it is VERY noticable.
3) IPS panels are notorious for input lag, unless you buy a Korean panel monitor with one of the bypass boards you will have horrible performance when it comes to gaming. Keep in mind input lag is different than response time, a 6ms response time is worthless if your monitor has 24+ ms of input lag.
4) If you really want better picture quality then get a dual monitor setup, one 27" IPS for daily use and editing work and one 24" 120hz with lightboost for gaming. The Asus 24" VH248qe with lightboost is currently the best performing gaming monitor available for under $300.
Lastly, if you really feel courageous the compromise is to buy a Korean panel IPS or PLS that has the ability to overclock to 96-120hz. For example, QNIX QX2710 or XSTAR DP2710 can be had for $300-350. They are 27" 1440p (2650x1440) and can overclock up to 120hz. The risk is two-fold:
1) The refresh rate you can overclock to isn't a sure thing, some of them overclock very well and some don't. The graphics card, cable and individual PCB all play a factor. Most people have no trouble getting to 96hz though.
2) These are A or A- panels stuck in cheap bezels, that's why you get a panel that would be found in a $1000 dollar IPS/PLS for a fraction of the cost. That means you are likely to have some light bleed and there is a decent chance of a couple dead pixels. The panel is not often set perfectly in the bezel which is part of the cause for light bleed. Often if you are willing to open the monitor up you can fix the light bleed (guides all over the internet), but you need to be careful.
My opinion? Go with the Asus VG248qe and call it a day, run at 1080p with a high framerate and enjoy the 120hz. Turn on lightboost (google how) and you'll virtually eliminate motion blur, it's the best gaming experience you can get. The colors aren't amazing since it's a TN, but you can always buy or borrow a calibration tool to take care of that. Most panels when calibrated are decent at least.
Edit: Don't bother with e-IPS, that is a 6bit panel and completely defeats the point of buying an IPS monitor. You get all the negatives of IPS without any of the benefits.
Edit2: Noticed you bought a monitor already but I stand by my recommendation of the Asus Vg248qe, at $279 it will destroy that Dell for gaming. That Dell has pretty severe lag which will make it horrendous for gaming, if that is your primary use I would strongly recommend returning it and getting something else.