i just can't believe the OP complain ISO 400 from a somewhat new T1i. especially you are printing 4x6.
i used to own that T1i, i use up to ISO 1600 all the time if i print 4x6, 3200 doesn't look that bad too.
i guess slight noise really bother some photographer.
Really! Go and get these shots à 800, 1600 or even 3200. You'll be blown by how much ambient light and atmosphere you'll get. I think that this is what you want from those party shots.
This will also use much less flash power and expand your battery's lifetime. Also, your owner's manual tells you that you shouldn't shot more thant 10 or 15 shots in a row at flash full strenght because it can overheat it or change the color of the flash glass cause of the heat... Another reason to put up the iso and open aperture as it requires less flash power and more safety shooting. Canon's flash don't stop by themselves because they're overheating, you have to be careful for that.
I agree with others, shooting in manual at 1/60 to 1/100, wide open to f/4, iso 800 to 3200, flash bounced off 90° or 60° and you will certainly have the results you want. I suggest TTL flash (metering Throught The Lens) and get the benefit of that 9-10ft roof. It will help you have a more diffused light on your subject as you know.
Also, turning your "to much grainy photo" in black and white gives an interesting result too. Kind of old looking photography. Give it a try!
Have a nice time