Alright, this is an interesting one. For starters, I have to say that I am impressed with the quality of this computer for the price Dell is offering; starting at $750. If you're looking for a prebuilt, this is probably a good one to go with. For starters, it comes with the i5-3450@3.1 GHz, but you can choose the i7-3770@3.4 GHz. It's a $150 price difference, but I feel that the one included in the price should be able to handle most GPU support processes without any problem. Of course, if you need more that I am not aware of, the i7 is definitely an upgrade, but depending on your budget, it may or may not be worth it.
It comes included with 8 GB of DDR3 RAM@1600 MHz, so apparently Dell has stepped up the game by providing 1600 speed as their new standard... who knows. Anyways, for most games, such as Battlefield 3, Skyrim, etc. RAM doesn't really make that big of a difference, but for video editing, multitasking, more RAM isn't really a barrier. Again, you can double that amount with $150, but you can also go in between with $75. I really think if you need any upgrade at all, 12 GB would be fine because that is already pretty much at the high end of the consumer grade computer RAM.
To begin with, the GT 640 and AMD 7570 are both low end cards from both companies. Remember that even though they are the new generation of GPU's, the AMD 75** are supposed to be an improvement on the 65**, and are not supposed to compete with the cards such as the 6950, 6970, 6770, etc. I'm pretty sure that this is the same for the Nvidia Cards. Honestly, it depends what kind of games you will be playing, because even though the cards are low end, they CAN handle most modern games at decent frame rates, maybe around 40-50 max. However, if you are looking for longevity (relatively) and improved performance, the additional $150 towards the 7770 would definitely be a vast improvement with another gigabyte of video memory. Otherwise, I would either stick with what it comes with. End price should be around $1000, but I'm not sure what your budget is!!