If you upgrade your Atlantean to mythical sword, the damage will be 120-150 for lowest ed 200%, and 140-175 for perfect 250%ed. The weapon's only real big bonuses would probably be the +2 pally skills and 50% to attack rating, but imo there are lots of better alternatives. You'll get more physical damage out of them, with lots of elemental damage also, and a lot more bonus to attack rating.
Nord's Tenderizer has similar physical damage, however it will also add 150-180% to your attack rating and has massive cold damage and good speed.
Baranar's Star has a good balance of elemental damage and great speed, with one of the highest attack rating % bonuses, along with + str and dex, which Atlantean also grants.
Save the upgrade runes, and instead get one of these maces. There are other great weapon choices too, but these two would be good alternatives that shouldn't be too hard to obtain.
you could upgrade the atlantean but as sjw said, there are better alternatives. instead why dont you consider upgrading an etheral dimensional blade to phase. put 6 sockets in it and save it till you can afford botd runes to make an etheral phase blade botd. for druids who are into melee i think its the godliest weapon possible, and can fetch a high price if you find the right person. hope this helps.