the sandy bridge pentiums are pretty good with basic gaming they are pretty much i3 2100s without graphics or hyperthreading, and they have a slightly slower frequency
i recomend this combo from newegg. http://www.newegg.com/Product/ComboDealDetails.aspx?Ite...
this processor is only 100MHz slower than the G860, and 200MHz slower than the 2100. if you get this you will also need new ram. but get 4GB of ddr3 1333 for under $40 and you will be set.
im assuming that you have a decent video card in your old system. that motherboard in my link does have graphic connectors (dvi, hdmi, vga) but unless you get a processor with graphics build in(2100 or higher), you wont be able to use them.
for some games you wont notice a difference between having a dual core processor or having a quad core. but newer games are always coming out and they are doing better and better at taking advantage of 4 cores.
if you want to go quad core the phenom IIs are still decent for quad cores, the better ones are cheaper and overall faster than intels dual cores. you could also go the way of a intel 2300 or higher but your looking at around $180 for the processor alone.