16GB(ddr3) is plenty, it's just isn't compatible with the newer boards(ddr4).
If you could get a good deal on a used 2600(k) or 3770(k), then that would be the next best option without changing the motherboard and ram as well.
Some of these are still floating around the retail price, which is BS. A cpu that old shouldn't be selling for much more than $100.
And for the retail price of those, you could get a new cpu/mobo/ram and be much better off.
I wouldn't pay more than $120 for one, and heck no for $150 - that's about half retail price!