My System (Also mentioned in my profile)
MoBo: GIGABYTE GA-990FXA-UD5
AMD Phenom II X4 955
Two XFX HD-577A-ZNFC Radeon HD 5770 (Juniper XT)
8GB RAM – Two sets of G.SKILL Ripjaws Series 4GB (2x2) 240-Pin DDR3 1600 (PC3 12800) F3-12800CL7D
Dlink pCIE X1 Wireless Card
Creative SB XiFi Xtreme Audio Card
Lite-On Sata DVD+-Writer
CPU Cooler – CoolerMaster Hyper N520
Windows 7 64 Bit
Dell 24” IPS LCD Monitor
NZXT Gamma Case
your temps look fine. if its only happening when your running 2 cards then its likley 1 of them is being starved of air causing the fan to run faster than it normaly would to get the same amount of air across the card.
if your front fan/s are variable ie you can turn em up and down at will, try turning them up to max while leaving the back fan at low rpm this should even out the air preasure in the case and hopefully give better cooling thus less noise.