My pc cannot play sims 3 expansions properly - which pc should i buy to play?

Is this what you have now ?

These are my usual  suggestions on what to get

Processor: i5 or i7 with at least 2.5 Ghz (don't get an i3, they have overheating issues)

RAM: at least 6-12 GB RAM, the game can't handle more than 4 but it's always good to have some spare for the graphics card, the OS and other stuff

Graphics Card: Very important. Get something that is green across the board on here: http://simswiki.info/wiki.php?title=Game_Help:TS3_System_Requirements

Operating System: A 64 bit system is a must

Hard Drive: at least 500 GB