My task is:
External link to internet at speed 50 mbits/sec which usually is loaded at about 80% (40-42 mbit)
Also, 3 local interfaces connected to different local ip subnets and NAT at outside interface to single ip assigned by internet provider.
The interchange between local interfaces is not needed, the only need - the internet connection.
Whether will enough use RB/450G or it is necessary to use RB1000?

RouterBOARD 450G should be enough for your requirements. RouterBOARD performance tests,
http://www.routerboard.com/pdf/routerbo ... _tests.pdf

Unfortunately, when I change the Linux router (Sun Netra T1 + Auror Linux - based on Fedora) with new RB450G, I see that effective loading of internet link decreases to 14-15 mbit and found packet loss on WAN interface about 20-30%.
The config of RB450G is very simple - only 3 ip addresses assigned to internal interfaces and external ip address assigned to eth1/gateway interface with NAT on it by default without any filtering.
So, I had to replace a router back.
What can I do to increase throughput on WAN interface?

Unfortunately, when I change the Linux router (Sun Netra T1 + Auror Linux - based on Fedora) with new RB450G, I see that effective loading of internet link decreases to 14-15 mbit and found packet loss on WAN interface about 20-30%.
The config of RB450G is very simple - only 3 ip addresses assigned to internal interfaces and external ip address assigned to eth1/gateway interface with NAT on it by default without any filtering.
So, I had to replace a router back.
What can I do to increase throughput on WAN interface?

interesting,
how much CPU used when RB450G was doing traffic?