A laptop is an already built PC -there are hundreds of possiblities.
If u could narrow ur search - what CPU/GPU/RAM etc. u want i could help u.
Newegg.com is an excellent site to buy any kind of hardware from.
Remmember the most imporatnt part of ur PC, by far, is the graphics card. Buy a 256MB card for around 150-200$ and the Sims 2 will run perfectly.