In my experience, it's generally ~3.25GB that will often be usable (the rest is lost due to addressing reservation space and perhaps other stuff), but it can vary up or down some depending on the configuration. Windows should tell you how much it is "detecting" in the System Properties.
I'd honestly get a new OS. Windows XP and 32-bit are time to be retired on newer setups. If it's got 8GB RAM, it either has no business having a 32-bit OS, or it has no business having as much RAM. So, if you won't be doing that, drop down to 4GB RAM and do whatever with the rest. It's being wasted sitting there doing nothing. You're already going to lose some with 4GB as it is, but as I imagine these are four 2GB modules, there's no harm in having 4GB.