I bought one more slice of 2gb ram (Kingston KVR800D2N6/2G) which is an exact replica of the one I owned previously. Ran into a problem when trying to install it.
First I put both of the rams in my only 2 slots and as a result of that everything (fans, hard drives...) turned on fine, except monitors were receiving no signal.
So I tried each ram individually - both worked fine in slot 1. In slot 2, however, neither of them worked (no signal, again).

So basically, can't use both of my rams at the same time. Is the 2. slot faulty? How do I test it?

Motherboard is Asrock G31M-S and the manual says it supports my RAM.

Tried resetting CMOS.
Also tried unplugging most of my hardware, so it's not a problem of power shortage.