I'm not sure what the monitor type has to do with being an MMO'er. The main advantages of IPS panels are viewing angles and colour accuracy.
My initial thoughts so far are, if I get a new gaming rig then I will need to get a new monitor. The problem I have encountered is if I primarily play MMORPG's, RPG's and Dota 2 then should I buy a TN or IPS monitor? A review sites asserts that the Asus VG248QE (http://bestgamingmonitorden.com/) is one of the best. However, the trade off TN monitors have don't seem worth it for a MMORPGer. Also I absolutely adore my current monitor which is a 4 year old Samsung which cost 100 pounds. Is the extra cost that noticeable?
Viewing angles are really only of consequence if you have a group of people watching a screen at different angles, such as watching TV. Normally, it's just you sitting directly in front of your monitor, so viewing angles, imho, are a non-issue.
Colour accuracy is only important if you do any professional graphics work or you're the type of person who can tell the difference between green #440 and green #441. Unless you sit two monitors (1 TN, the other IPS) side by side, for most average computer monitors, you won't see the difference.
Well, that's strictly up to you, really. The system you have is pretty close to the standard 1920x1080 gaming rig. You'd have to get a Haswell i5-4430 (plus new motherboard) and a GTX-650 Ti just to match what you have. A new system with an i5-4670/4770 and a GTX-760 would be the minimum to get any significant benefit.
Secondly .. my current rig is 2 years old and runs all the games I have thrown at it so far.
Perhaps, depending upon just how bad her laptop is, you might be better off just to put together a low end gaming rig for her. Something like a basic A10-7850 system. WvW and large events would have poor fps, but the rest of the game should run fine, especially if you settle for a 1600x900 monitor.