i know for me it takes a bit for it to smooth out. and you'll run into certain area's that lag a bit more.
i havn't played it for a while but im pretty sure i got everything maxed out and its very playable except for a few slowdowns in a few spots
im running I7 920 with 6gb of corsair and a 4890 ati....but i remember playing this game on a 6800 GT and a amd 3000+ which was like a 2.1ghz or something
oh i know what you should try first, you need to go download the vista direct x driver
(rename your current one so you can easily swap it back) the file is called dxdiagn.dll
that file causes alot of problems with older games on windows 7, if your running the 64bit version this file will be located in syswow or w/e its called
oh and you might have to try a few different versions of that dxdiagn.dll file, otherwise i could email you the one im using. i just know that one of the ones i downloaded did not work. if your wondering where i found this out, it was regarding NWN2, which wouldn't load at all because of this file, so maybe just maybe it will help with your lagging
this may or may not fix your problem let me know