Hey man, check out pcpartpicker.com its what most of us use here on the forums to get an estimate of everything.
If your looking mainly for a gaming pc, you'd want to get a good video/graphics card over anything else...however, with $400, don't expect a really good pc. (although I've heard stories of people building these magically awesome pcs under 400 bucks, but I have never seen one)...anyway, check out the site.
So, 1366 is an out-dated socket, and trying to find a good mobo to go with it is difficult, especially with your budget. So what I am going to do is have two builds, one with your cpu, and one without.
CPU: Intel Pentium G850 2.9GHz Dual-Core Processor ($64.99 @ NCIX US)
Motherboard: ASRock H77M Micro ATX LGA1155 Motherboard ($69.99 @ Newegg)
Storage: Seagate Barracuda 500GB 3.5" 7200RPM Internal Hard Drive ($52.99 @ NCIX US)
Video Card: XFX Radeon HD 7850 1GB Video Card ($159.99 @ Newegg)
Operating System: Microsoft Windows 7 Home Premium SP1 (64-bit) ($92.11 @ Amazon)
(Prices include shipping, taxes, and discounts when available.)
This is difficult. The second system includes a far superior graphics card, but it takes a step down on the motherboard in terms of features. However, the second mobo is an H77 and supports the new ivy class of processors. The main comparison to be made between these two are the CPU's. To help you out, I will include a benchmark comparison of the two. http://www.anandtech.com/bench/Product/47?vs=404
In gaming, they are fairly close, but the 920 destroys in in multi-threaded applications. I'm not really sure what you value more.
Regardless though, if you stick with your CPU, your are buying an out-dated motherboard, and if you ever decide to go with a CPU upgrade, you will have to buy another board.
If you get a new board, at least you will have an updated build that will allow for a CPU upgrade.
Personally, I would wait until you have 500-600 dollars. At that price range, you can get a really nice system.
I hope this all helps!
While I prefer intel CPUs, you could always get an AMD APU which has a decent quad core CPU and graphics in one chip, an A10-5800K is 3.8Ghz with a 7660 GPU, which should play most games ok, and it's only $130.
There is a fairly basic concept I am going to explain. If you are gaming, a half decent dual core paired with a powerful GPU is always better than a powerful CPU paired with a weaker GPU. More or less, spend more on the GPU than the CPU.