pretty old technology for $400.ddr2 ram is about $50 for a 2gb stick.the gpu is nothing special as well.i just dont find $400 here.
I dont know why people are apparently selling the Radeon X1900 XT 512MB video card for 300 dollars or so
Those people hope and know that there a people that do not know much about tech and just want to buy things fast without checking out what is really working and what it will cost in today's price for a video card.
You have intel i7 serie which have 4 cores and 8 threads (use wiki for more explanation) or Xeon E3-1230v3 which is an CPU for small server or workstation. Do not need to make use of ECC RAM. i7 do have integrated HD 4600 for video, but for more heavy gaming you would probably be better of with a discrete video card. With Xeon E3-1230v3 you will need a video card as it does not have HD 4600, but there are version with this also from E3-1245v3 and up.  in this SKU here means HD 4600 support.
There YouTube with some people playing LoL on intel HD 4600, so it should be playable.
AMDs FX firstname.lastname@example.org or FX8350@4.0 GHz do not have integrated video and need VRM that is strong enough to provide power to these 8 core CPU on mainboard chipset. Best MB for those CPU will be 990FX or 990X chipset.
There is also the latest kavari (A10-7850K) which is an 4 core cpu with rather strong integrated GPU power. Google for benchmarks for each of these suggestion and you will get a better picture of what is best for you.
In any case, it will be hard to build a system for 400 dollars, but it will at least be working for a couple of years from now if you upgrade it gradually after you have put it together.