My RB450G has just arrived.

The task is simple, NAT for a few PCs.
Which are the steps that I should follow?
Do I need to update anything?

think this is what you need
http://wiki.mikrotik.com/wiki/NAT_Tutorial

You need to configure IP addresses (http://wiki.mikrotik.com/wiki/Manual:IP/Address), possibly a DHCP client on your WAN link (http://wiki.mikrotik.com/wiki/Manual:IP/DHCP_Client), possibly a DHCP server on your LAN link (http://wiki.mikrotik.com/wiki/Manual:IP/DHCP_Server) and NAT (http://wiki.mikrotik.com/wiki/Manual:IP ... Source_NAT).

Since you're looking for a simple setup, the wiki should answer all your questions.

Thanks, fewi.

Do I need to update anything? firmware/routeros/etc?
I think I've read somewhere, a while ago, about a time limited update or something like that.

If you feel like upgrading to the latest version, you can. The Wiki has a how-to guide. However, for just a NAT'ing gateway you're not going to gain anything by doing that. Unless you're hitting a specific bug the functionality you want is so basic you might as well keep it as is.

Is there a period within I have to update?
Or I can update anytime I want?

RB450g comes with default configuration:
* dhcp-client on ether1
* ether2-ether4 switched together and running dhcp server
* masquerade on ether1 interface
* basic firewall rules to protect the router

So for simple task as you described in first post you don't need to configure anything, just plug the cables in right ports.

Thanks for the tip, mrz.

I've just connected to the board via WinBox.

The ISP provider that I have forces me to use an application that tries to stop NAT-ing(long story short, they'll drop it soon due to large number of petitions).
Anyway, the application can be tricked until they'll drop it:
-the WAN interface of the router has to be set with specific IP/GW/SM/DNS
-the ether2/3/4/5 ports have to use IPs from a specific class, eg. 85. ... ... ...
-TTL has to be increased by 1

Another question, what does Bad blocks from System->Resources mean? It says 40.

And another one, how do I increase TTL by 1?
The ISP sends the packets with TTL=1, in order to block some entry level routers.

And another one, how do I increase TTL by 1?
The ISP sends the packets with TTL=1, in order to block some entry level routers.

Assuming your ISP is coming in from port ether1: