There are no real problems for 64-bit OS currently, in the past I know there were big driver issues. Just about everyone that is good with PC's on the interent is getting the 64bit version, the only reason to get the 32bit version is if your making a home theater PC.
If you intend to still use the Z5500 speakers you should be able to use a digital input from the MB and there would zero quality difference between the MB sound and a sound card. Personally I've setup the Z5500 for a friend and they were good for movies but not very good for music.
the p6t7 is definitely overkill for a gaming computer and would not show one extra frame when gaming, it would only be for someone who likes to burn money. Gigabyte also makes good MB's, the GIGABYTE GA-EX58-UD5 would be a good choice. The p6t6 is a good choice but a little overkill too.
The power supply you picked seems to be an older model, so thats why I was suggesting the newer standard. But I guess the 80+ standard must be a USA thing. If you pick one that is in that link I put in the above post if should be good. If you unsure of how good the model is just ask us here.