How may noise voltage and noise figure be compared?
At 50 Ω, noise is \$\frac{E^2}{50} + I^2 \cdot 50 = -146\$ dBm/Hz;
\$174-146= 27\$ dB noise figure at 50 Ω.

You don't need to consider the 5kHz, it cancels out. At low impedances, you can ignore In, at high impedances you can ignore \$V_n\$. (relative to equivalent noise resistance)

For an amplifier with \$V_n\$ and \$I_n\$ given, you can consider the equivalent noise resistance \$R_n = \frac{V_n}{I_n}\$. 

This will be the impedance where noise figure will be lowest.
In this case 15kohm. 

As you see, it is very noisy at 50 Ω.

Noise power is \$V_nI_n=-171\$ dBm/Hz . Thermal noise is -174 dBm/Hz, so the noise figure would be 3dB at 15 kΩ.

The numbers are only valid at low frequencies, at high frequencies they are different, so you can't really compare.