Hi, I was wondering what the resolution of my screen depends on? is it something to do with my graphic card? or is it my monitor, or both?

Both. Your monitor will have it's limit, but it may take a particular card to reach that limit. (especially on older computers).

Start > Control Panel > Display > Settings will show & adjust your resolution.
In my example you see the monitor & card:

1. NEC MultiSync LCD1700V on ALL-IN-WONDER RADEON VE PCI

I could not reach that high resolution before I installed that card although the monitor supported it.