The firmware is version 4.01.37 and should only be used on version 3 of the DG834 and DG834G routers. Note: Do not use this firmware on North American models.
The firmware fixes a number of issues, but what seems to be the major one was mentioned by Zen Internet on our forums.
Whether you should upgrade your DG834v3 firmware is a personal decision, a general rule is that if the connection with the current firmware is working fine then leave things well alone. Those who stand to benefit the most would appear to be those suffering frequent disconnects which hopefully the tweaks to the DSL code will improve.