Two consumer GPUs share the same headline capacity but lead to different local-AI decisions. The Radeon RX 7900 XTX and GeForce RTX 3090 each provide 24GB of dedicated memory, so a given model file has the same nominal file-only margin on either card. That does not make them interchangeable.
The capacity tie is real
Using the official Qwen2.5-32B-Instruct-GGUF repository, Q3_K_M is displayed at 15.9GB and leaves 8.1GB before cache and runtime allocations on either card. Q4_K_M is 19.9GB and leaves 4.1GB. Q5_K_M is 23.3GB and leaves an unsafe 0.7GB. Q6_K at 26.9GB and Q8_0 at 34.8GB do not fit wholly on either 24GB GPU.
Software breaks the tie
RTX 3090 uses NVIDIA’s CUDA ecosystem. RX 7900 XTX requires a verified AMD-compatible path such as the exact supported ROCm release, operating system, driver, framework, runtime, backend, kernels and operators. A CUDA-only application does not become compatible because both cards have 24GB. Windows, WSL and Linux support claims are not interchangeable.
Condition and measurement finish the decision
Many RTX 3090 candidates are used, so repair history, memory stability, fan behavior, hotspot delta, connectors, warranty and return protection can outweigh a lower price. For performance, hold the model artifact, quantization, context, cache, backend, runtime version, prompt/output profile, concurrency and timing constant. Record unsupported operators, fallback, offload, peak device memory and complete-system wall power rather than inferring a winner from core counts or gaming charts.
Use the full comparison
Compare RX 7900 XTX and RTX 3090 · Check CUDA versus ROCm · Calculate complete VRAM · Inspect a used card · Submit a controlled result · Stage a private watchlist
Published October 25, 2026. HighVRAM has not independently benchmarked this pair. Official specifications and file-size arithmetic do not establish application speed, output quality or value.