When you do the drive don’t forget your system is only a SATA II (3.0 Gb/s) system so make sure the drive you get is able to run at this speed.
Depending on the size of the drive you get it can get quite expensive. Given the age of your system I wouldn’t put too much into it won’t support macOS Mojave (10.14) so High Sierra is it for this puppy.
Frankly, if you are running El Capitan or newer you should max the memory out to the full 8 MB this system supports before doing the drive.