If fan remake Black Mesa was too much of a departure from the source material for you, and you feel the urge to revisit Gordon Freeman's 1998 debut, this sharpens things up without making drastic changes to the art.
Check out the comparison screenshots below to see the mod in action. In some areas the change is barely noticeable, but overall everything looks a lot crisper, and not in a way that overwhelms the OG aesthetic.
The textures in Xen remain unchanged, sadly. But honestly, when you jump through the portal in the Lambda Core, quit the game, fire up Black Mesa, and play their version instead. It's much better.
Make sure you expand the screenshots (using the icon at the top-right of each image) to see the differences. They're subtle, but they're there.