DEFINITELY do not do Vista 64 for gaming, like ginnal said. Hahah, "does not compute"... well said
I tried Vista 64 on a Dell XPS M1710 and had to trash it and go back to Vista 32. The laptop does mostly business apps and occassionally some light gaming.
Biggest issue with Vista 64 is still driver support. Yeah, most devices are supported now... but they tend to run older drivers. Vista32 and XP are still getting the most driver updates.
If you're doing it for RAM's sake... well, you'll get an extra .5 or .75GB of RAM depending on your hardware, but IMHO it's just not worth it. Buy 4GB of RAM and be happy with the 3GB you get to use.
For now it's enough. If you are going with 6GB Vista 64 makes more sense but I think 6GB would be mostly wasted right now (e.g., just used up as Vista pre-fetch memory and not really "used").
PS, you can always go Vista64 later. If you buy Vista retail I am told it includes the 32bit and 64bit media. So if it were me I'd just install 32bit for now and when things mature or you decide you need that extra bump of RAM go with 64.
Functionally 64 was not any faster than 32 for me and just was not worth the headaches.
Oh, for sound card... The SoundBlaster X-Fi series is very popular. I recently ditched my X-Fi and installed an Asus Xonar D2. It works great for gaming, provides excellent sound in Bioshock and Quake Wars so far.
I would avoid X-Fi personally if you are connecting this to a home theater receiver as the X-Fi cards do not do Dolby Digital Live or DTS Interactive (which basically encodes your multiple surround streams into a single toslink or coax connecter, vs. doing individual analog outputs). Some people claim the analog outputs sound better, but I strongly preferred coax/toslink.