Good retailers/distributors of pre-built PCs will usually give you lots of customisation options, give you deals on certain upgrades, let you choose not to have any bloatware at all, and actually be cheaper than building your own PC unless you hunt for really good deals on each part all over the place. They're also no less upgradeable than a custom PC AND they often come with (even if for a small price) a money-back or replacement guarantee. I had been upgrading my PC for 6 or 7 years, I think, and then enough parts were so old that I finally decided to start from scratch. So glad I did.

I suggest you get the GTX 560. I've got a GTX 580, and it's awesome. I normally go mid-end, though, and I'm always glad I did (I just got a really good deal on the high-end rig). And more than that: the lower end cards of each generation (such as the 520, I imagine) are usually severe under-performers -- they're cheap, but slow. You'll almost never used the advance features they technically support, because their low performance means those features are often impractical in an actual game.