The game should work fine on Windows XP (with SP3). OpenGL (unlike DirectX, ironically, don't get me started on that but this article pretty much nails it if you're interested:
http://www.extremetech.com/gaming/133824-valve-opengl-is-faster-than-directx-even-on-windows) is fully supported so there's no issue there.
As for system RAM, the engine is very efficient so it really just isn't a problem. It could use a full 4GB with a 64-bit OS or 3GB with a 32-bit OS but even 2GB should be plenty. Adding lots of characters or objects etc. wouldn't change that, as @
Nubby rightly pointed out you'll hit the limits of your CPU long before addressing memory becomes a problem. VRAM usage is higher and could potentially be very high (with extreme/future settings or mods etc.), but that's handled by the graphics driver/OS, not the application, so you can already use as much as your system will let you.
Something to note about texture resolution is that it is there is a limit to how high it should be, based on the resolution of your screen and the size/distance of the object from the camera. While in a first person game there's almost no limit to how much resolution can improve the appearance of textures, this is not the case with our view. The textures we use are already very high resolution and we've taken a number of steps to further improve the quality that would be very difficult to replicate. Some of our textures are procedural and somewhat limited in resolution (e.g. the terrain) but because they are procedural this is down to engine systems and modders would not be able to simply increase their resolution, they could still however modify their appearance. Bottom line, you're unlikely to have much need or use for higher res textures and therefore more VRAM.
