I do seem to remember the 8800GT normally making fan sounds that weren't constant. Kinda like a wurrr-slowdown-wurrr-slowdown-repeat, whilst under load in a game. (with normal nvidia drivers and no tools)
70 idle... hmmmph it would ofc depend on the rest of ur rig's situation, but I would have expected 60-something cos I don't remember these cards being that cool.
I'm basing this ofc on the assumption that the 9800 is more or less the same 8800 with only some pcb component differences.
The cards may look slick with the full on black single slot look, but the cooler isn't the best imo.
Temps at full load would be the thing to keep an eye on. 110 is too damn hot (imo), 80/90 is probable. Being at 70 idle doesn't leave much scope for increasing.
Heck if it's new(ish) replace it, u got nothing to lose there.