Nginx uses the ssl_ciphers directive to select ciphers.[0]

    ssl_ciphers ALL:!ADH:!EXP:!LOW:!RC2:!3DES:!SEED:RC4+RSA:+HIGH:+MEDIUM;

KeepAlive is enabled by default, but you can configure it for different durations or numbers of requests.[1]

I believe you can chain your certificates by concatenating the chain to your certificate file.[2]

    cat chain.crt >> mysite.com.crt

[0] http://wiki.nginx.org/HttpSslModule#ssl_ciphers

[1] http://wiki.nginx.org/HttpCoreModule#keepalive_disable

[2] http://wiki.nginx.org/HttpSslModule#Synopsis