Hi, GACLove14. Looking at the Q&A section of the site, the computer has a 350w power supply, which is barely adequate for the components. It will run okay, until you run something demanding on it, then it could easily overdraw the power supply. So, I would recommend getting the updated power supply. You can replace the power supply without voiding the computer's warranty, or you could pick up a decent power supply at Best Buy and have Geek Squad install it if you're not comfortable fiddling with the internal bits, heh.
As for the difference between the two computers, the $600 model has more memory, more hard drive space and a better processor. The graphics card is about the same. The power supply is also the same, so you definitely want to replace the power supply at the moment you purchase it, or very shortly after - don't run Sims 3 or Sims 4 on it until you replace the power supply. EVGA and Corsair are great brands for power supplies with good reputations for quality parts.
For the monitor, it is more of a personal choice - what may look good to me may not look all that great to you. Most monitors will work pretty well for games as they support 60 Hz refresh. I would recommend looking at the monitors at Best Buy and see what kind of color and picture they have. You want a monitor that has decent looking blacks and color that doesn't smear or bleed at object edges (what is called over saturation). Another option is an inexpensive LCD TV makes a great monitor.
Unless you are getting into high-end gaming, you shouldn't need to worry about Ultra HD monitors or 144 Hz refresh rates. In this case a simple full HD display is the better option (1920 x 1080 - referred to as 1080p). As for touchscreens, there are several models available. I don't know about in-store availability, but you can find several online. Whether touchscreen or not, some good brands are Dell, ASUS, Acer, HP and ViewSonic.
As for TVs, if you want to go that route, Vizio and Samsung make great displays. Samsung is pricey compared to Vizio, but their picture quality is excellent. I use a large-ish Vizio TV as my computer monitor and really like it.
Hope this helps and happy Simming!
Flynn
