Definitely build your own to save money, and it's fun. For $700 you can build one hell of a machine especially if you don't need windows as well. It's nothing that the two thousand dollar overclocking enthusiasts would get excited about, but it will definitely run modern games on high/ultra with good FPS.
There's a wealth of information out there and forums in which enthusiasts are more than willing to help. Research and ask questions! I built my own just a few months ago and the process went like this:
1) Set yourself on a budget (Started with $550ish ended up at $650 due to errors -- had to repurchase my PSU and didn't know I needed a new windows)
2) Confirm the parts you will need. For me it was motherboard, case, graphics card, processor, new hard drive, RAM, power supply, and a new copy of Windows.
3) Divide those into profitable/non-profitable price-performance groups, and a non-variable "set price" group as well. In this case, the graphics card and processor would be where I need to invest the most money and I would see the most performance increases. The motherboard/Case/RAM/hard drive are important, but will not impact performance as much. If you use windows there is only a set price so put that in your budget -- you should not go cheap on the PSU so consider it a set price as well (what you need and how much you pay will depend on your set up).
In other words, we want to drive down the price of the motherboard, case, ram, and hard drive while investing as much in the graphics card and processor as we can.
4) Scout graphic cards/processors and get a vague idea of what you want (Brand, socket etc) and if you are going to overclock.
5) Look up your compatible motherboard and ram configurations, and then start looking for deals. Find within reason the cheapest board/ram/hdd/case you can and then try to find a good deal on your processor and graphics card.
6) Confirm the power you will need, and see if a trusted and safe PSU will fit in the budget with your set up. If it doesn't and you don't want to go over, readjust your graphics card/processor combo, check the requirements once more (graphics cards can vary greatly on power draw) and try to fit the PSU in again.
Go to reddit's buildapc and have them check it out! Viola. If you approach building your PC with the steps above it will really simplify the process and keep you on budget while building a good rig.
Also I highly recommend pcpartpicker dot com and going to toms hardware and checking out the quarterly builds they have for pcs at a certain budget.