You could use either OpenCL or HLSL/GLSL -- Those fancy Parallax Shaders are a form of ray-casting for example.
However, more to the point, as you said yourself, Games like Delta Force: Land Warrior did this stuff on CPUs way back in the day -- I recall running Delta Force at relatively low settings on a Pentium 2 @ 233Mhz -- and Commanche did it even earlier. I'm pretty sure I've seen a similar, though limited, voxel terrain running on either the Gameboy advance or some microcontroller as a tech demo.
It sounds to me that you simply need to dive into optimizing your code -- A single modern CPU core ought to be able to push modern, full-screen resolutions if you can achieve similar efficency to those old games (and I bet a really optimized one could do perhaps twice the frame-rate at the same time). Modern CPUs have 2, 4, even 6 cores (Discounting AMD's "8-core" Bulldozer, but are slower) so there's absolutely no reason you shouldn't be able to get satisfactory results.
I imagine you'd learn more by making your CPU algorithm fast, than by porting your slow CPU algorithm to OpenCL or shaders.