I wonder if...

[Cutie]

A reason might be virtual memory. Thats one of the reasons for me. Or if you have CC. Honestly, I have around 10,000 in my CC folder so I get complete slowness &.&

But you shouldnt worry about what to do with TS3 since you have almost a year (or perhaps 2 - for sure, not before 09) to think about it and prepare   

And we wont know if the Sims 3 has the same requirements as the Sims 2. So its hard to replace certain parts of your pc if you dont know what you have to fix  ;D