And he said MT2 is a crap DOS-Converted program for windows
I'm afraid your dad has no idea about what MT2 is. MT2 is a pure native 32-bit Windows application.
It is also a multi-threaded application, so in a way you'll benefit from the dual/quad-core, but not to its fullest as the main thread (the audio engine) is not split up in several threads. So the raw clock speed of your CPU is important with MT2. The RAM is too. If you had 256 MB before with XP, it really wasn't enough. You clearly should see a difference with 1 GB or RAM.
In general, you will always benefit of dual/quad core since it makes your computer always responsive whatever happens. If MT2 is using 100% of CPU, it'll only be 100% of one of the cores. So the rest of the system will remain very responsive.
Dual/quad core is also very interesting if you're often using multiple applications at once.
So if you can't fully enjoy the quad-core with one application, you'll always benefit from it thanks to the extra comfort and stability.