The new RX 7900 XTX versus RTX 5080 local-AI comparison addresses a common buying fork: 24GB of Radeon memory versus a newer 16GB Blackwell card with CUDA.
The 8GB capacity difference changes what fits
The official Qwen2.5-32B-Instruct Q4_K_M file is 19.9GB. It is larger than the RTX 5080’s 16GB pool before runtime overhead, while a 24GB RX 7900 XTX leaves 4.1GB of raw file-size margin. The official 15.9GB Q3_K_M leaves only 0.1GB on 16GB—an unsafe planning margin—but 8.1GB on 24GB. Context, KV cache, runtime workspace and display use still count.
Capacity is not the same as speed or compatibility
The comparison does not convert CUDA cores, stream processors, clocks or bandwidth into a cross-architecture speed claim. The Radeon path requires an exact ROCm support check; the Blackwell path requires current CUDA-capable software. Reference board power is close at 355W versus 360W, but the published system recommendations differ at 800W versus 850W and partner boards vary.
Use the CUDA versus ROCm checklist, compare other 16GB paths and 24GB paths, or run the hardware-fit calculator.
Contribute a matched result
Stage both exact hardware configurations while holding model revision, artifact, context, cache, runtime settings and timing constant. Record the OS, driver, CUDA or ROCm version, unsupported operators, fallback, exposed and peak memory, CPU offload and wall power. You can also watch both families privately in your browser.