What memory type (1066 / 1333) should I buy for the AMD X4 FX-4100 3.6GHz processor? It's 3600 MHz with FSB of 2000 MHz as far as I know. Is it still relevant to match memory and cpu frequencies?
No, it's not relevant.
It really never was. It's true that a 1:1 ratio helped efficiency a bit on some configurations, but a 5% difference in speed trumps a 2% difference in efficiency any day, so faster is still better.
And it's certainly not relevant now. Your AMD X4 FX-4100 doesn't even have an FSB. The HT bus doesn't connect to memory, but to the crossbar. So there's nothing to match.