Can I run Spark-X2.6-4B-FP8 on an RTX 3090?

Yes

Its published weights (4.45 GB) fit in the 21.6 GB an RTX 3090 can use, with room for ≈ 453K tokens of context.

hf download XHToken/Spark-X2.6-4B-FP8

Files and context on RTX 3090

Each file of Spark-X2.6-4B-FP8 (and of GGUF versions the site has analyzed), and how much context fits next to it, with the context cache in 16-bit or 8-bit.

Spark-X2.6-4B-FP8 on other hardware

Popular models that run on RTX 3090

Assumes 90% of the card's memory is usable and at least 4K tokens of context for one conversation, with the context cache in 16-bit unless noted. Fits leaves at least 20% of that memory for context; Tight leaves less.