Welcome to the forum!
What Wiked basically pointed out is that if you build your own computer, you simply just can't call tech support if some issue arises with your build. However, if you have a problem and buy quality name brand parts, you can always call support from the manufacturers, but usually they won't completely help or solve your issue. This is where tech forums like MPC will come and help out. Honestly, you can basically find most answers to simple tech problems on the net; plenty of helpful guides and solutions for anything, even for complicated issues. Will you understand it is another question.
There are times to where it'll be better to buy a pre-made computer, but that usually involves these points:
1. If your budget is very very low, thus the price for a completed computer with warranty makes sense even though you get cheap components; it'll work and cheaper than if you build yourself (remember, they buy parts in bulk and get windows software for pennies). Typically, if your budget is $500 or less and your needs are minimal, then pre-made wouldn't be a bad idea to look at, specially when you can find a complete system for that price from the big names.
2. If you just don't want to hassle with building one yourself (above points), but its super easy.
3. If your technical building skills comes into question for high end systems with lack of knowledge and experience. I'm talking about pricey computer systems like from Origin PC, AVA direct and such. Sometimes its better to direct people of less building experience who have money to burn towards a pre-built gaming rig with a warranty, instead of them taking a chance and ruin their $4k parts because they forgot to tighten down the hose clamps on their liquid cooler loops...
4. For people who just shouldn't be around ANY tools... not even a screw driver... You know what I'm talking about, you've seen these people. People who are a danger to themselves and people within a 100m radius. lol
Well beside those points, its super easy to build a starter gaming rig. With $700, you can build a pretty decent rig. Here's what I listed that you could do:
This nets you a pretty powerful setup. I was hoping to budget in a 6870, but those graphic cards went up in price by $30. The x6 cpu is still a powerful chip and a really great all around chip from AMD. Don't even bother with their new FX chips, not worth the silicon even if that silicon is in some hot chick... heh I paired the cpu with a nice Asrock (Asus btw) motherboard that has some good features; the only thing that's lacking is SLI for nvidia graphics cards, but since I stayed with AMD for this build, it doesn't matter.
Hard drives are still kind of priced high and I'm sure you could find some better deals then spending $90 for a 500GB drive. I'd stick with 7200rpm as your main boot drive instead of the cheaper 5400 or 5900 green drives you hear. They will work and you basically get double the capacity, I'm more in favor of performance and speed in this regards. You can bump it up to a 1TB drive and find them around the $110-150 price points for a better price/GB ratio, but up to you if you want to spend money in that area. I think 500GB is the bare min now and just wait till HDD's normalize later in the year if you need more space.
Slapped in a nice but cheap Corsair 430w power supply paired with a nice looking and decent ThermalTake case that says good value. You could always look for better quality cases in the $50-90 price range, but this is basically the bare min. Finally add in a cheap $16 dvd burner and Win 7 64 oem and your done.
Other small tidbits to look at getting is a tube of Arctic Silver 5 thermal paste for the cpu, maybe 1-2 120mm Rosewill case fans for your... well case, and small putty/thermal paste spreader.
MPC has some great guides for building computers, plus lots of other guides on tom's hardware, anandtech, extremetech and loads of other places. There's no shortage of How-To's in this area.