Because If we (several wireless clients) download too much (that means "full speed" for several hours) the router (WRT160NL - OpenWrt 10.04 with two default 8 dBi omni antenna) just reboots randomly. Or are there any "watchdog" like solutions built-in OpenWrt so that if the router has too many load for too long time it reboots it?
If I understand this right are you asking if big antennas make it reboot or little antennas? Confusing. I'm still not sure how the antennas come into this question. If you do have wireless clients that are getting slower performance through different antennas then obviously there is less that they will be able to load up the broadband. Although this depends on the speed of your broadband too. Normally the broadband has less bandwidth than the local wireless network.
BTW, are you downloading with bit torrent?
I'd say your router is rebooting because it's filling up it's memory. You should be able to tweak the conntrack settings (tcp timeouts in iptables) in OpenWRT or reduce the maximum number of open connections? It's probably a good idea to limit the number of connections per local PC.
I'm not sure how you do this with OpenWRT but it's very easy with Tomato firmware. The options to do it should be available.
You could also try a different firmware or firmware version.