Dude, if all you want to do i splay CS:S, then you could probably spend less than $200 and still have a PC that's complete overkill. PC Games are generally made on the specs at the time the project is started, so when it is released 2 years later, you could say it is already 'outdated', in a sense. CS:S is an old game, made older by this fact.
Don't get me wrong, I'm not trying to bash it or anything, I actually got it (kinda funny, my PC couldn't handle it 5 years ago and when i built my new pc I was finally able to redeem that $30). My point is that it doesn't take much to play 'today's games, the PC in my sig can play Crysis 2 on maximum with no problem, max fps. The issue with pc's is how will they last.
I recommend you build the PC I have in my signature, but replace the GPU with a GT430 or a Radeon 4830 - whatever you can get in a combo deal better. A 4850 would be complete overkill.
As for a CPU, the Athlon II X3 450 is just the best processor for the money right now. If you want to overclock, the 440 or 445 or even 435 may be better if you willing to gamble, although it is cheaper, and there really is no reason to go above a X2 Regor for your build.
I would suggest just getting a combo deal on newegg for Athlon II X2 + Motherboard, or X3 if you really want to 'splurge'.
The PSU you picked out is complete overkill. Get the Antec Earthwatts 430D. This power calculator is the most accurate I've ever seen, as calcs like neweggs just want you spend money. You could also be thorough and calculate on your own how much power you need, but nah, why would you want to learn how PC's work
But seriously, the Antec Earthwatts series is the best for the money right now. Unless you are BOTH overclocking and doing SLI/Crossifre 2x GPU, there is no reason to go above 400w PSU's. However, you do want quality - you need to make sure the 12v rails on the PSU have most of the power.
As for case, NZXT Gamma is a better case for less money.
Your CPU choice isn't really good - the Athlon II X3 450 is $10 less but sacrifices L3 for an extra core, and the current revision from newegg is generally said to be unlock-guaranteed. Black Edition is generally a price premium for no reason at all - to do a thorough overclock, you have to raise the FSB and lower the multiplier anyways, so you'll probably not even go past stock FSB. If you're too lazy to overclock correctly, then don't do it. You can always just raise the FSB on it's own until the system limits you, and be happy with the gain there - it will be just as much as raising any multiplier on it's own.
I think people have pretty much made clear on this board that Gskill's are bad RAM, and RMA them a lot. Just get the cheapest RAM you can, as what is most important is the size, not the speed or timings. 4GB is all you'll ever need to do all the multitasking in the world. I recommend you get 2 x Kingston HyperX Blue 1333mhz 2gb, they have been shown to be better quality than those Gskills and overclock very well. You can check benchmarks online, but the difference between the fastest RAM in the world and the slowest DDR3, is less than 10%, while costing 10x more. If you are intent on 'not buying crappy RAM', which isn't true since Kingstons are absolutely amazing, you are better off spending the money on a better CPU or GPU.
HDD is simply a matter of how much space you need. Just make sure it's 7200RPM and SATA.
here's a GPU that's twice as good as the one you picked, for the same price by the way:
Anyways, you should get the GT430 in a combo deal, a 4830, or 4850 at most.
As for you're motherboard, why are you getting such an expensive motherboard? If you have plans to overclock, you need to buy smart, not expensive, and I'd be willing to talk about that. Otherwise, just get a CPU+Motherboard combo, as all the recent newegg AMD CPU + mobo combos are good quality.