Am I going to be OK just with onboard sound these days?
Unless you are a complete audiophile: yes, it will be more than sufficient. One thing to consider: If you end up with surround sound speakers, you may want to look at a motherboard that has the 6 connector jacks instead of the 3. either one will work with all the speakers, but if you have a 5.1 speaker set it will take up all three jacks leaving none to connect a microphone to. (Which may be an issue if you also want to do voicechats like Skype and the likes)
As far as the 64 vs. 32 bit is concerned: I run vista 32 at the office, and vista 64 at home. Speed wise, there is little apparent difference. some 64 bit apps get a marginal speed boost, but in reality the biggest difference is that under Vista 64 you can address much more memory than under 32, and that 64 has mandatory driver signing. (Which means that not all of the low-end or older stuff has drivers available to it, which is bad, but also that there is a little more quality control and thereby more stable drivers. (Most of windows' instabilities are driver-related)
Now, 32 bit programs run through WOW64 (windows-on-windows 64) under Vista64 -- there is a translation layer, so possibly a very marginal slowdown, but I have never noticed a difference at all in day-to-day use.
One important distinction between 32 and 64 bit: Under 32-bit windows, there is WOW32, that allows you to run 16 bit programs. This no longer exists under Vista64 -- you can either run 64 bit programs natively, or 32 bit through WOW, but no support for 16 bit at all. Most people have no 16 bit programs left, but you never know so it's worth mentioning. We're talking about windows 3.x era software here. (Oh, including the powerBASIC for windows compilers, the actual compilers themselves are/were 16 bit as well, at least not too long ago)
Anyway, I like Vista64 better than the 32 -- the 32 version keeps bugging me that it's running low on memory and that I need to close some programs as soon at I use 2.1 GB RAM (3GB installed). It's amazingly easy to hit that 2.1GB in use threshold, vista definitely is a memory hog. It will then keep freezing for short moments, apps stop responding, etc. Under Vista64 I have 6GB, and never gotten any complaints or slowdowns.