@Sin: 1080p is not bad advice. That's the most used resolution at the moment, and it's probably the highest resolution her video card will be able to push on modern video games (not that she plays any of them). Also the monitor she uses now is a 4:3 piece of shit, nothing close to 1080p. I'm pretty sure 1440p would be overkill for both ET and whatever non-gaming things she does on her computer (Freecell, web browsing, etc.). It'd be a waste of money IMO.
@Winnie: I only mentioned 3 different numbers: resolution, refresh rate and screen size. I can't see how screen size and resolution can confuse you, and I already explained what refresh rate did. But anyway, here you go: A higher refresh rate means your monitor will be able to refresh itself more times in one second, thus allowing it to display more frames per second. So games that run at a frame rate high than 60FPS, such as 120FPS or 144FPS, will look smoother. Your ET probably runs at 125FPS so if you get a 120Hz monitor you'll definitely notice a difference between that and your current 60Hz monitor. Screen size is exactly that, the size of your monitor's screen. It's measured diagonally. And resolution is measurement of the amount of pixels your screen can display. The more pixels you have, the more space you have to work with, and generally the better games look. Here's an illustration of different resolutions, so you can get the idea:
http://techgage.com/wp-content/uploads/2013/01/4k_comparison.png