Hi Harkonenn,
You can go from a 4200 RPM hard drive to a 7200 RPM hard drive. You won't have any problems as the internal PSU will supply the hard drive with necessary power. What is important is that you pick up and IDE (ATA-6 or ATA/100) hard drive and not a SATA hard drive.
These would be examples of hard drives compatible with your laptop:
http://www.newegg.com/Product/Produc...ubCategory=380
Just a note, the Hitachi hard drives are just as good as the Seagate hard drives but often more expensive, so you should look at those too.
Feel free to post back with any other questions.