help to choose a vga fan cooler

hello, i am a newbie, i have a evga 7800gt, and want to change the fan to zalman vf700 cu or vf900 cu. i only want to make it a little quiter, not for OC. is there a big different between these two fans and worth to spend a little more bucks for vf900 cu? if anyone knows other fans that are around 30 bucks and very quiet, please give me a recommandation.
also, if i remove the original fan and heatsink, does this gpu still under evga's warranty? thanks.

sorry, i have very bad english, b/c i was a chinese...not Communism..... :x 8O
tnaks again..

I had one of these on my 6800. There cheap and quiet plus they pull hot air out of the case. I wish I could have gotten one for my 7900 but they are too long. I currently have the Zalman 900 and am very pleased with it so it would also be a good choice but is more expensive than the artic cooling.

http://www.arctic-cooling.com/vga2.php?idx=40