Well if those games a single core is enough.
And you should know that AM3 semprons are actually AM3 *regor* Athlons II x2 with a core locked. You can get a cheap mobo, unlock the 2nd core and get a nice improvement. Also an small overclock, lets say about 3.2ghz will let you play all those games pretty well.
I must add that everything i wrote can happen if you are extremely lucky to get a fully funtional core and stable unlock.
But if you are able to unlcok an adiontal core, there are chances that it comes bad and only gives you problems as stability or undesirable heat.
The best recomendation we all can tell you is to get a cheap motherboard and a very budget card. Spending $150 i guess is the very top you should go.
That mobo and that GPU are kinda good, I can suggest this GPU which is much better than the one that you listed http://www.ascent.co.nz/productspecification.aspx?ItemID=403732
That motheboard looks kinda good you can use it, but for me i would go Asus or ASrock, also looks like that Gigabyte doesnt have Core unlock feature
! Well... just for your knowing, that a single core PC wont be for gaming, unless you play old games.