The RTX 4090 24GB has about 121% more memory bandwidth (1,008 vs 456 GB/s). Batch-1 decode tracks bandwidth closely, so a model that fits both cards should decode roughly 121% faster on the RTX 4090 24GB. The RTX 4090 24GB fits more of the catalog at Q4/8K — 33 vs 18 of 61 models (24 vs 12 GB VRAM). Largest model that fits the RTX 4090 24GB but not the Arc B580 12GB: GLM-4.7-Flash (31.2B). MSRP is $249 for the Arc B580 12GB vs $1,599 for the RTX 4090 24GB. MSRP is a launch list price, not live retail — Amazon prices move; click through for the current price. In the Q4/8K head-to-head below (40 shared models), the RTX 4090 24GB posts the higher tok/s in 37 rows vs 3. Methodology.