I am also having 4GB of ram and running proudly x86. The other 0.5 you mentioned are stored as a non-paged memory/whatever and are used in extreme cases for speeding up your system.
Also i do not think you play games or use programs that drain 3.5GB of ram at once and you can't work on your PC because of the lag. But still glad we solved your problem, this is what matters the most. 
Not this misinformation again.
32-bit addressing can only cope with 4GB of RAM. That 4GB must include your graphics card RAM and other bits and bobs throughout your system. That means that no, the other 0.5 isn't used at all, it's not addressable by the system in any way, shape or form.
What this means is that if you have a 512MB graphics card, you'll get around 3.25GB of accessible RAM, although the exact amount depends on the motherboard and other resources.
The single exception to this on a 32-bit system is using the PAE (physical address extension) which allows 36-bit addressing, but it off by default and can cause conflicts with device drivers. It also doesn't work well unless you have 8GB or more of RAM in the machine, using it with 4GB is inefficient due to the way PAE works.
This is all well-documented in multiple places across the 'net from both hardware manufacturers and operating system programmers, there's not really an excuse to
still be getting this information wrong.
Additionally, there's no significant compatibility issues with using a 64-bit OS. There's no visible slowdown in games or general headaches.
I've been running 64-bit versions of Windows for over 2 years now and not come across
a single problem that was down to the OS being 64-bit. Indeed, the only ones you're likely to are those that do things like hook into the kernel, such as software firewalls or AV, for which there should be a 64-bit version around for anyway.
Anyway, rant over.
As to fixing AS in this instance, Vista and Win7 have an application called 'start' that allows you to set options like this, put the attached .bat file in the audiosurf directory and running that should auto-set the affinity for you. Just make sure Steam is running first.
