Short answer: Both cards provide 24GB of dedicated memory, so the same model file has the same nominal file-only margin. The decision is mostly software support, exact workload behavior, used-card condition, power and price—not capacity. RTX 3090 is the safer default for CUDA-only software; RX 7900 XTX can be a viable ROCm path only after the exact operating system, driver, ROCm release, runtime, framework and required operators are verified.
Do not choose from core counts or gaming charts. NVIDIA CUDA cores and AMD stream processors are not directly comparable. Published clocks, memory types and architecture labels do not establish local-LLM token rate, image-generation speed, output quality or value.
Official platform comparison
| Official specification | GeForce RTX 3090 | Radeon RX 7900 XTX | Planning meaning |
|---|---|---|---|
| Architecture | NVIDIA Ampere | AMD RDNA 3 | Software backends and supported kernels differ |
| Memory | 24GB GDDR6X | 24GB GDDR6 | Same nominal capacity; allocation behavior still depends on the stack |
| Memory interface | 384-bit | 384-bit | Equal interface width does not prove equal application speed |
| Published processing units | 10,496 CUDA cores | 6,144 stream processors / 96 compute units | Counts are architecture-specific and must not be normalized directly |
| Reference boost | 1.70GHz | Up to 2.5GHz | Partner boards and sustained clocks vary; cross-architecture clock comparison is not useful |
| Reference board power | 350W | 355W typical board power | Measure complete-system wall power under the same workload |
| Reference system power | 750W PSU | 800W minimum PSU recommendation | Confirm the exact board, connectors and the rest of the system |
| Reference size | 313mm × 138mm; 3-slot | 287mm; 2.5-slot | Partner designs vary; inspect clearance and airflow |
| Primary software gate | CUDA/runtime and driver support | ROCm/backend, OS, driver and framework support | Compatibility must pass before performance or price comparisons matter |
NVIDIA values describe the RTX 3090 Founders Edition/reference design and its official user guide. AMD values describe the RX 7900 XTX reference design. Add-in-card specifications vary.
The same 24GB file boundary
These official Qwen2.5-32B-Instruct-GGUF repository display sizes produce the same nominal file-only arithmetic on either 24GB card. They exclude KV cache, runtime workspace, display use, modality, concurrency, allocator behavior and fallback.
| Official artifact | Displayed size | 24GB file-only margin | Planning result on either card |
|---|---|---|---|
| Q2_K | 12.3GB | 11.7GB | Capacity fits; quality and full allocation require testing |
| Q3_K_M | 15.9GB | 8.1GB | Useful starting point; calculate cache and workspace |
| Q4_K_M | 19.9GB | 4.1GB | Capacity-sensitive; calculate the complete run |
| Q5_K_M | 23.3GB | 0.7GB | Not a safe resident target |
| Q6_K | 26.9GB | Does not fit wholly | Requires another placement, offload or more device memory |
| Q8_0 | 34.8GB | Does not fit wholly | Move to another artifact or capacity tier |
Repository display values checked October 25, 2026. Download size is not peak resident memory.
Choose by software first
If the application, extension or model path requires CUDA, the RX 7900 XTX is not a substitute merely because it has 24GB. For AMD, verify the exact current ROCm compatibility matrix, operating system, driver, ROCm/runtime version, framework, backend, quantization, kernels and unsupported operators. Windows, WSL and Linux are not interchangeable support claims. For NVIDIA, verify the application’s supported CUDA/runtime and driver versions rather than assuming every older Ampere setup works unchanged.
Used condition can reverse the value decision
RTX 3090 is commonly evaluated used, while RX 7900 XTX listings may be new or used. Compare delivered price only after checking exact board identity, repair or mining history, fan behavior, memory stability, hotspot-to-core temperature delta, connector condition, warranty and return protection. A cheaper unstable card is not better value. Use the used-GPU checklist.
Run a controlled comparison
Hold model revision, quantization, runtime version, backend, kernels, context, cache precision, modality, prompt and output profile, batch, concurrency, timing boundary and repetitions constant. Record exact board, OS, driver, CUDA or ROCm/runtime versions, unsupported operators, compilation changes, fallback, offload, exposed memory, peak device memory, system RAM, power limit and wall power. Failed or CPU-fallback runs are evidence, not results to hide.
Choose the next evidence
Inspect the RX 7900 XTX fit guide · Inspect the RTX 3090 guide · Compare the 24GB tier · Check CUDA versus ROCm · Calculate complete VRAM · Run a controlled workload · Submit a reproducible result · Stage a private watchlist · Check screened observations
Sources
- NVIDIA GeForce RTX 3090 official specifications
- NVIDIA GeForce RTX 3090 official user guide
- AMD Radeon RX 7900 XTX official specifications
- AMD ROCm current compatibility matrix
- Official Qwen2.5-32B-Instruct-GGUF repository
- HighVRAM methodology
Reviewed October 25, 2026. HighVRAM has not independently benchmarked these two cards head to head. Specifications, file fit and software support do not establish application speed, output quality or value.