I have the original one WRT54G but if I was going to get one now, I think I would buy the WRT54GSUK.
DD-WRT is a firmware which can add extra features to your router like increasing the wireless signal making the router more reliable... etc...
But... if you did flash the router with the DD-WRT firmware, your warranty will probably be void...
More about the DD-WRT firmware
here.
I recently upgraded my router to this firmware and definitely recommend it.