When it comes to future proof, FM1 is out of the question, as FM1 will NOT be compatible with upcoming Trinity.
LGA2011 is out of question for you too since motherboard and CPU will exceed $300 budget limit.
Thus you are left with AM3+ and LGA1155. First, you need to know that next year Intel will be using LGA1150 for Haswell. That however, also means that Sandy Bridge and Ivy Bridge will be cheaper in used market. You have plenty of options for LGA1155.
Upcoming Vishera will be using AM3+, after that it is very likely that AMD will use another socket for Streamroller.
You can try to grab Phenom II X4 960T, 970 motherboard, and DDR3 combo well within $300.
You can also grab Core i3-2100, a Z68/Z77 mATX motherboard, and DDR3 combo well within $300 too.
Advantage of 960T is being a quad core CPU which means better performance on multi-thread/multi-tasking. Gaming performance will surpass i3-2100 when you OC 960T past 3.6 GHz.
Advantage of i3-2100 is easier to find, newer CPU design, power efficient, better performance at SLI/CF, and better gaming performance at stock. You also have the option to buy SB/IB i5 down the road which will blow AMD's offering overall clock to clock.