First, a note - anything installed with Wubi is going to be slow, due to the way Wubi works.
That said, it might be worth running a diagnostic of your hard drive, if the install was also slow. Very often a major slowdown, especially in a clean OS install, is indicative of a hard drive going bad. I recommend Hitachi's Drive Fitness Test, Hiren's Boot CD, or the Ultimate Boot CD.
There's a difference between graphics-related slowness and slowness related to other issues. Generally, graphics slowness is lag in repainting actions, such as changing screens or scrolling through long documents. If you see this kind of slowness, too, make sure you've installed the Nvidia drivers. You can do this by first updating your computer (open Unity and search "update"), which also updates your video driver list. Once the updates are done, open the "Software sources" application and go to the "Additional Drivers" tab. Find the Nvidia driver that says either "stable" or "stable, tested" and select it.
On a final note, your specs are more than enough to run Ubuntu with Unity, Gnome-Shell, or KDE (also, KDE is on par with Unity as far as resource usage, so it's not surprising that going to Kubuntu didn't change anything), but if you want something lighter weight, then you might want to try LXDE (Lubuntu) or OpenBox. Also, the terminal isn't generally required in most cases for the general user anymore, but even when you do make use of it, it's not that bad (and who knows, maybe once you get used to it, you won't want to go back to some of the GUI interfaces ;) ).