From my knowledge, you should stay away from any Geforce FX cards, because they´re as fast
slow as Geforce 2 cards and Lunia is unplayable with these. And your nForce 405 is not a gfx card, but the chip, so stay with your current card. Lunia should run fine.
Regarding the "lag": Most people complain about two different types of lag.
1) FPS lag if their computer can´t run the game properly
2) Internet lag, because ijji´s server got mad (very rare) or because of their poor connection to the server.
To find out if you lag, you execute a tracert from your system prompt in Windows. There you can see your expected ping and if any packets are lost.
tracert -d ijji.com
I´m from Germany too and my ping is around 180ms. There´s a small delay but most of the stuff you encounter won´t annoy you.