There's never any real reason to go from Mage to fighter because you miss out on hundreds of hit points.

If you don't have ToB (it comes out here next week-can't wait) you want to swap from Kensai at either 9th level or 12th level. If you go at ninth you max out your mage levels while still getting enough weapon proficiencies to grand master in your chosen weapon. If you go at 12th you get extra combat abilities and a few more hit points.

I usually only go at 12th level if I'm dual classing two fighter type-one goes at 9th and the seond at 12th-gives the first new mage the chance to get established.

Incidently a pure fighte/mage works extremely well too because you can use a shield whe you exceed your fighter level with the mage ones.

I will be getting ToB (only reason I have not yet is just so I don't get too tempted to play it before I get this new char going hehe)
Most likely I will want to develop my fighter skills more than level 9 so I will probably wait for level 12.

If you have the patience to play through the whole of the BG2 game, you'll get a pretty nasty bit of work - a longsword called Blackrazor. My Kensai-Thief uses Longswords as a Grandmaster, and has also points in dual wield, and Katana.

For R-P reasons, it's kind of strange to see a Katana and a Hammer on the same character. Longsword/shortsword and dagger make a better visual combo, as do dual shortswords. The Eastern Katana and Wakisashi idea is a good one too.

I like this weapon(Blackrazor) so much that I'm going to restart again this evening, with my assassin(now at level 21) and maybe dual class her to a fighter for TOB. It regenerates you slowly, which is very helpful for a solo game.