I always thought that having a good graphics card and a good monitor would make my drawings worse. Why?
With the monitor - because if it changes the way things are displayed, you may not be able to see some of the things that will make your drawings uglier, and as such - you won't be able to correct those mistakes (because for you - they don't exist, as you can't see them). This usually happens when the monitor you work on displays black and dark grey colours very closely to reality. When that happens - it's hard for your eyes to see the difference between the specific shades and thus makes committing a mistake much easier. As an example - can you see a difference between the following pictures on your computer: EXAMPLE ?
This point still holds true - a well setted display helps you to see your work for what it really is. If your monitor is too good or badly adjusted, there's a chance your work will look well on your monitor, but badly on others'.
Secondly, the graphics card. I always believed that having a good graphics card works well for the games and videos (and 3D rendering, obviously). But I thought that it makes your drawings uglier in the same way that the monitor does. In other words, I believed that a video card makes your drawing pretty on your computer, but ugly on others', if they don't have a card as good as yours.
That, I now believe, is actually not true. The graphics card changes the very way you DRAW your work, making it better on the general level. It works a little like the stabilizer in some graphic programs. Simply put - the computer uses more power to make a line you draw on your tablet as close to what it would be if you were drawing on paper. In other words - it eliminates as much of the mistakes that usually go with using a tablet instead of a normal pen - as possible.
Ever wondered why some artist's works are so beautiful and yet look as if they've taken them so little time? Well, maybe they just have a better computer than you do, and can draw with it just like they do on paper - without having to use eraser every-second-1-milimeter-long-stroke to make it look showable. (Of course this has nothing to do with the actual skill - it just means how much less difficult it can be to draw on computer compared to the paper.)
To conclude - I believe having a strong computer with a good video card makes your life easier as a graphic designer, but you have to be careful with choosing a monitor, and make sure that you can see all the colour differences on it accurately.
PS. Also, getting a stronger computer works much better than changing your tablet. With the same tablet and a new faster computer you can really feel the difference. Unless your tablet is almost dying, I suggest you get a better computer instead.
(Tl;dr: Don't make a mistake like me and get yourself a good strong computer with a great video card and don't spend your life with an eraser as your second lineart brush!)