This is a predominantly BIY community, so many of our red flags went up immediately given that you were buying a prebuilt. You'll have to give us a few ideas of what you want to run on that, and at what kind of resolution. Don't be discouraged, your PC will be able to handle a large amount of titles; the GT 520 gets more hate then it should because it doesn't do so well with the benchmarks, but it will get you through some solid gaming if you realize its limitations and account for them. First, get comfortable with
system requirements lab, it is extremely useful for determining if you have the minimum stats to run a game, but should be taken as more of a guide than absolute rule.
The greatest part of owning a PC for gaming is your ability to upgrade, and when you finally get to that point (don't worry about it now, get a feel for your machine first and decide if you need more power or if you can wait), the first upgrade you should get is the video card. Depending on what your PSU is (get a detailed sheet from your seller on the parts, and hang on to it), your options will vary since most GPUs nowadays require additional power from a special connection from the PSU. Three cards to consider, depending on what price you can find them at: HD5670, HD6670, GT440. These should be your starting point, any lower than that, and you're really limiting yourself for how much you'd be spending. Put aside at least $80 to get started, and after switching, realize that you could probably get 30 or 40 bucks for selling the 520 you're taking out.
The CPU will have more gaming viability (my brother was running Company of Heroes, Fallout 3 and NV, and even The Old Republic on a Sempron 140- far less than what you have), but after a graphics card upgrade, any increase in performance will be limited by the CPU. Skip the triple cores, you'll pay upwards of $70 for a net single core increase, best to just pony up $100 and get a quad. As wr6133 said, the AM3 platform is now inferior, so CPU production for it is being phased out. Finding a good price on an upgrade will take time, so watch the market before pulling the trigger. You'll want to move up to a Phenom 955, but even an Athlon quad core, if you can find a reasonable price on it, would be worthwhile. Don't go for a 6-core, you won't need it for gaming.
If it were me, depending on how well that GT 520 performs (hardware forums tend to dislike it, but it gets good reviews at retailers; I don't have one so I'm on the fence), I'd just go with your set up for a few months. You'll want to upgrade eventually, but if you don't mind it just stick with it for a while; absolutely take advantage of any deals you see when Thanksgiving rolls around, that might be a good time to upgrade.
Now that we've dealt with the hardware, make accounts on Steam and gog.com; these are my recommendations for gaming software. They both have ample sales (and you'll want to wait for them on Steam: I bought Oblivion GOTY for like $6), Steam has many demos so you can try out games/test your hardware, and gog has DRM-free games, usually dated a bit but still fantastic. Newegg is my preferred hardware retailer for parts and reviews, though amazon and tigerdirect are swell as well; amazon manages to beat newegg a lot in both price and availability, so always compare. Tomshardware and anandtech are where I like to read up on the tech; toms does a monthly "best CPU, GPU for the money", and it is an incredible article. I'd suggest at least reading the bottom and second to bottom category every month just to get a feel for where your potential upgrades lie.
I'll leave you with my thoughts on where you should be able to start, based on benchmarks. My laptop has a 2.9GHz dual core with HD 4250 graphics. If I punch my hardware up and tweak the graphics settings, I can play Fallout 3 (medium settings) easy; Company of Heroes (low settings) easy; Civ 5 (low settings) on small maps decently; Civ 4 (high settings) on medium worlds, or I'll dial down for larger worlds; Left for Dead 1 on medium settings; and Skyrim barely (like at 800x600, it looks worse than Morrowind (which runs great on high btw) at this point, but it's playable and smooth- mostly to say I can). We'll assume that both our GPUs are the limiting factor, and most benchmarks put your GT 520 at twice the results as my HD 4250, so it stands that you should be able to get at least these but mostly increased results on your system.
/I'm a bracket whore
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