Windows XP is just using 2.5GB of the 3.5GB installed. Is there a simple way to see which PCI driver allocates how much of memory? I'm looking for something that shows me this data on a single screen (I don't want to click through the PCI drivers properties pages and manually copy the addresses).
You can use the device manager for this. Just open it (Workstation -> Properties -> Hardware -> Device Manager) and then change the view (View -> Resource by Type).
Then it'll list how much RAM is allocated for every device. Free RAM is a big chunk named "Mainboard". Note that there are duplicated entries, so if one of them doesn't really make sense, check the address of the next one. If they overlap, then those two entries are for the same thing and one of them should have a meaningful name.