IIRC speeds are

USB 2.0 Interface ---> 150 to 300 MB per second depending on your PCI bus

SATA Interface ---> 1500 MB per second approx

Hard drive speed is above USB2, so yes, external USB2 drive is slower. You may not notice the difference however, as it will be affected by your other components and the nature of the data being copied.

However I'm worried that you have 1.5 GB of RAM. It's a weird amount. None of the "big boys" in computers (Dell, HP, etc) will ship a machine with a 1.5 GB in standard configuration as it'll cause some odd problems sometimes (mysterious crashes that people usually blame on Windows). Should have 1 GB or 2 GB.