Arc B580 12GB vs RX 7900 XT 20GB for Local LLMs

The RX 7900 XT 20GB has about 75% more memory bandwidth (800 vs 456 GB/s). Batch-1 decode tracks bandwidth closely, so a model that fits both cards should decode roughly 75% faster on the RX 7900 XT 20GB. The RX 7900 XT 20GB fits more of the catalog at Q4/8K — 32 vs 18 of 61 models (20 vs 12 GB VRAM). Largest model that fits the RX 7900 XT 20GB but not the Arc B580 12GB: GLM-4.7-Flash (31.2B). MSRP is $249 for the Arc B580 12GB vs $899 for the RX 7900 XT 20GB. 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 RX 7900 XT 20GB posts the higher tok/s in 33 rows vs 7. Methodology.

Spec comparison

	Arc B580 12GB	RX 7900 XT 20GB
Vendor	Intel	AMD
VRAM	12 GB	20 GB
Memory type	GDDR6	GDDR6
Memory bandwidth	456 GB/s	800 GB/s
TDP	190 W	315 W
MSRP (list)	$249	$899
Buy	Search Amazon (affiliate)	Search Amazon (affiliate)

MSRP is the launch list price, not live retail. Buy links are Amazon affiliate links — the price after you click is the live price.

Q4 / 8K head-to-head — top 15 of 40 shared models

Model	Params (B)	Arc B580 12GB tok/s	Arc B580 12GB fits	RX 7900 XT 20GB tok/s	RX 7900 XT 20GB fits	Winner
Qwen3-0.6B	0.8	362.7 est.	Yes	636.4 est.	Yes	RX 7900 XT 20GB
gemma-3-1b-it	1.0	290.2 est.	Yes	509.1 est.	Yes	RX 7900 XT 20GB
Llama-3.2-1B-Instruct	1.2	241.8 est.	Yes	424.2 est.	Yes	RX 7900 XT 20GB
Qwen3-1.7B	2.0	145.1 est.	Yes	254.5 est.	Yes	RX 7900 XT 20GB
gpt-oss-20b	21.5	180.6 est.	No	141.4 est.	Yes	Arc B580 12GB
Qwen3-30B-A3B	30.5	117.8 est.	No	169.7 est.	Yes	RX 7900 XT 20GB
Qwen3-Coder-30B-A3B-Instruct	30.5	117.8 est.	No	169.7 est.	Yes	RX 7900 XT 20GB
GLM-4.7-Flash	31.2	113.1 est.	No	169.7 est.	Yes	RX 7900 XT 20GB
Qwen3-30B-A3B-Instruct-2507	30.5	117.8 est.	No	169.7 est.	Yes	RX 7900 XT 20GB
Phi-4-mini-instruct	3.8	76.4 est.	Yes	134.0 est.	Yes	RX 7900 XT 20GB
Phi-4-mini-reasoning	3.8	76.4 est.	Yes	134.0 est.	Yes	RX 7900 XT 20GB
Llama-3.2-3B-Instruct	3.2	47.4 measured	Yes	131.2 measured	Yes	RX 7900 XT 20GB
gemma-3-4b-it	4.3	74.4 est.	Yes	130.5 est.	Yes	RX 7900 XT 20GB
Qwen3-4B-Instruct-2507	4.0	72.5 est.	Yes	127.3 est.	Yes	RX 7900 XT 20GB
Qwen3-4B-Thinking-2507	4.0	72.5 est.	Yes	127.3 est.	Yes	RX 7900 XT 20GB

est. = bandwidth-based estimate; measured = curated real benchmark that overrides the estimate. Fits = weights + KV(8K) + 1 GB overhead ≤ 95% of VRAM.

Fit coverage at Q4 / 8K

	Arc B580 12GB	RX 7900 XT 20GB
Catalog models that fit	18 of 61	32 of 61
Largest exclusive fit	—	GLM-4.7-Flash (31.2B)

Arc B580 12GB details → · RX 7900 XT 20GB details →