That makes it less likely but not totally. Windows 7 uses ram more aggressively for caching, it may be making more use of the ram than you normally did with XP.
But most likely is one of the default hardware drivers that is installed is not really compatible with the hardware you are using. The defualt drivers try to be generic and minimalistic as possible to cover the widest range of hardware possible without requiring 50 gigs of storage to do it
Updating drivers with the "real" ones from manufacturers web sites might help. (I actually had the reverse with my NIC though heh, had to roll back to the default one)