There is a lot going on that is just wrong in this thread. I think we need to redefine what the goal of it is.
Is the objective to run Crysis on a 23" (assuming 1920x1080 resolution, aka: standard 1080p) for $500?
To take it a bit further, is the objective the above stated but also run it with high or "ultra" graphics settings as well 4AA and AF?
If so I can save us all alot of trouble and newegg googling, NO it's NOT.
However is it possible to play Crysis and newer titles at "decent" graphics settings on a 23" monitor then the answer is possibly.
But here I beg the question... do you plan to upgrade your system that you intend to build to run the above or not?
If not, then by all means by the cheap motherboard that Xaira recommended along with the rest of the components.
However if you would like some sustainability out of your system I would suggest a different road.
First off there are some key components to a PC that not only add to the potential of a build but also give you options down the road you never would have thought of. One of those is the PSU. Sure you can spend $25 on a cheap PSU and run the risk of it frying you system or dieing in 2 months or you can invest in that component. A component that, given the right investment can last you 10 years or more. Same thing goes with a PC Case. A Case is something that can last your entire life time! Investing in that hardware now can prevent you from ever having to buy a case in the future. Sure you can buy a cheapo $25 case but chances are, like a cheap PSU, you'll have problems with it and when your system is done you junk it and have to buy a new one. But a little investment now can save you $100s in the long run.
So you have a budget of $500 and you wana run crysis at on a 23" monitor. Here is what suggest, not only can you keep your PSU and Case for your next PC, but your motherboard as well if you decide to upgrade within your platform.
PSU (power supply). This model is a 650 80 PLUS certified Active PFC unit. It's $30 more then what xaira suggested but you'll have it for many years to come and it will run CPU and GPU combos into the future.
http://www.newegg.com/Product/Product.aspx?Item=N82E16817371015
Current Costs: $80
CASE. The one thing you'll likely have forever. The Antec 300 series is perhaps the MOST proven case of it's generation. It's a no fail buy:
http://www.newegg.com/Product/Product.aspx?Item=N82E16811129042
Current Costs: $140
MOBO (motherboard). I can't in good conscience recommend a 700 series chipset so I'd go with this, more expensive again but far more support including 4x Ram slots instead of 2.
http://www.newegg.com/Product/Product.aspx?Item=N82E16813130295
Current Costs: $215
Next up the CPU and on this I have no disagreement it's a great bang for buck cpu. If you have $60 more to spend get a Phenom2 x4 955 if not this is the cpu for you:
http://www.newegg.com/Product/Product.aspx?Item=N82E16819103871&nm_mc=AFC-C8Junction&cm_mmc=AFC-C8Junction-_-na-_-na-_-na&AID=10446076&PID=3899435&SID=skim1402X558040
Current Costs: $320
The GPU, and important part of the system. The GPU is the biggest thing to consider in gaming. A 5750 is nice... but why not going nicer. The 4870 out performs a 5770 thanks to it's 256bit memory interface. It way out performs the 5750 and at only $5 more it's the best buy. You can always upgrade to a modern GPU later but for now you have a ton of horse power at a cheap price.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814121376
Current Costs: $450
Memory. Memory these days is very close to each other... just about everybody is offering a life time warranty. So budget is important, but so is upgrading. Here is where I push past the budget. It's only slight, but the overall investment of the complete system justifies it. Here you get your 4gb of RAM but with the option to upgrade to 8gb later without trashing any hardware.
http://www.newegg.com/Product/Product.aspx?Item=N82E16820134786
Current Costs: $537
In conclusion... I doubt there would be anybody on these forums who would argue (in absence of a HD and DVD/CD drive) that this is the a great system on the AM3 platform for around $500. This system gives you TWO fold your investment. It performs NOW! and it has RAM, CPU and GPU options to perform in the FUTURE. Please look at what your getting for your money. You can spend less sure... but half of what you get will be worthless in a year and the other half will be trash to begin with.