Another common thing: check if the motherboard isn't making contact (short-circuit) with the case! It is possible that by installing or removing the sound card the mobo now touches somewhere a metal part making some fuzzy connections.
floppy drive -> indeed classical
also try reseating the videocard, is it PCI or AGP?
can you test your psu with another mobo/cpu combination?
I can test the PSU with another mobo. I just flashed the cmos battery, waiting for 24 hours then ill put it back and see what happens...