Have pretty similar setup except I use the on board sound, an XP2100, one stick 512MB Corsair PC2700, Win2k and Dx8.1.
In flight I get 120 max which is top refresh rate for my monitor at the resolution l run. Sometimes it will drop to the 60-70s for a little bit in heavy action and yes once it dropped into the teens but I think some background crap started running.