Is that 9800GT 65 or 55nm? Pretty good temps there.
OP: As long as you don't have any issues using a dual slot cooler, e.g. a PCI/PCI-e 1x slot being blocked, then I believe there is no valid reason NOT to get a dual slot cooler. They are most
often quieter (mostly because their thicker fans can push more CFM's at a slower speed), and cool the card better (more metal surface area for heat exchange).
If price is equal, dual slot would probably be better. I would feel more comfortable if my 8800GT had a dual slot cooler on it rather than the single slot. I just know the single slot cannot cool
the same card as well as dual slot can at the same noise level. You will see some guys on here list the exceptions. "My 8xxx single slot runs 35C idle and 59C at load".
This may be true, but in a cold room. Variables play a part.