Ok guys I did my own research. It seems the best I can get is a GT640. Found a guy selling it cheap here on Lowyat. I play mostly 2D games so it should suffice for now.

According to Dell site, the PC supports GT640 with no additional PSU. Has been tested.

However, that version is 900mHz non-OC. The one I am getting is slightly OC to 940mHz. Is this effect on the power output minimal, or noticeable? Do you think I shouldn't get it as a result?

I know, but it is around 3 times better than my GT620 in two separate benchmark scores I have seen. My GT620 is also actually fine enough for now for 2D games, and I rarely if ever play 3D games. Just want something that bit better.

Depends, in the very simple 2D games, zero lag. In the more intense 2D games with bloom effects for example, or in 2.5d games (sidescrolling but with 3D-ish backgrounds), I do have lag and sometimes need to even go down to 1366x768 to play at decent framerate.

In any case, my CPU should not be a problem for the foreseeable future as I have a 3rd gen Core i5 and also 8GB ram.