I don't think so, even though its the same card technically, I really doubt the drivers would work with both cards, its like saying the 8800 GTS will SLI with the 9800 GTX.... I doubt it will work. Unless you flash the 9800 GT into a 8800 GT, but thats just not worth it.
Apperently what I read was that there are 2 9800 GTs, 1 55 nm and the other 65 nm.
ivbeen told that you can mix 8800gt with 9800gt but they will only run as fast as the slowest card. but since they are practicly the same card anyway. its a good point about driver surport tho! i serpose you must have to use both 9 & 8 series drivers. it should work! afterall your sound card drivers dond get mixed up with you're ethernet card drivers for example. so why not.