For a roleplaying game, it sold well enough. That it doesn't sell as much as a Sims expansion pack, pff, those things sell 58million copies a day. David Gaider seemed well pleased with the sales of KotOR for PC on their boards, but yes, obviously it didn't sell as well as it did for the Xbox (which shattered all records). However, considering that it was the work of six months to do the port for the PC version, Bioware was very pleased with the sales numbers for that selfsame PC version.
True, the sales charts don't mean a whole lot when over half of the games are Sims titles. However NWN stayed in the top 10 for 3 months in comparison to the two weeks of KotOR. IW hasn't breached the top ten but for all we know it could be howevering at 11 and thus still be doing quite well for itself. I guess we'll just have to wait for sales numbers to be released.