I'm using Ubuntu 8.10 with on an old computer which is not very powerful, it has no WiFi on the mother board. I would try to use an USB interface WiFi and I've seen many interfaces on the market but nobody have the explication if it is possible to use the hardware with Linux. Please, can you suggest me some HW or a way to resolve the problem? I would like to connect the PC to internet. Thank you very much.
Ubuntu 8.10 is no longer supported and you will no longer receive any updates.
You'll have to do the work yourself if the 8.10 kernel doesnt support your device i.e. find out what the kernel module is required for the device, compile it and add it to your 8.10 kernel.
If you are worried about newer supported versions of ubuntu requiring more resources - then look at installing Lubuntu or Xubuntu. Both are lighter on resources - Lubuntu itself has a minimum requirement of 128Mb RAM.
By using a newer version of ubuntu, you will more likely have a kernel that supports your device out-of-the-box or that is more likely to be used by people here on AskUbuntu or on UbuntuForums.
For finding out what devices are compatible with Ubuntu - look at Ubuntu Certified products. If you install 11.10 Lubuntu or Xubuntu then most modern wifi USB devices will be supported - sometimes you will need to install firmware, but Ubuntu usually takes you through this process via the Additional Hardware window