Ps2 slim takes 8.5v. Theres talk that some of the Japanese ones take 7.5v, but Ive never seen proof. For a ps2 slim, try to get one in the 790xx range, as the motherboard is much smaller, and has lower power draw as well. (lasts 2x longer on my batteries than my other slim). A perfect power solution for this is the power board out of an Initial rb-270 DVD player battery. It takes 7.4v in and spits out a very steady 9v. Nice thing about it, is that you can replace the cells (18650's x6) with other ones. Has an integrated smart charge circuit, and also an LED capacity gauge thats pretty accurate, no matter how many cells you add. (Ive gone up to 16 18650's on one board, and it remained very accurate, since its voltage based). Biggest downside is heat generation, but if you get airflow, it stays plenty cool, and replacing sinks is easy.
As for the SNES, depends on if its a slim, original, or clone. I know a slim runs off a single 7805, so it has a max of ~1.5A draw.