This seems a direct comparison to the card I'm using right now - and that card you posted seems better.
So... then it comes down to what you're looking for. Realistic terrain, dust and shadows, or just a fast enough card to handle the game well. I'm fine with my card
, and I can win some battles with it, so... I'd be more than fine with that GT 640 card you listed there.
oops... didn't read that part of your post - you already addressed the power supply
Dell, and others, seem to build to the lowest specs possible these days.
Yeah... you have a very basic computer if that's yours. I'm no super-wattage power-supply fan (I ran a 350 watt power supply for two years with this card... but there is no doubt that the new power supply I just put in the system this month made it all more stable and smoother). Yours says it has a 250 watt power supply... great for a gaming computer from ... 1998 or 2000, but that's quite a bit low, IMHO, to expect it to run your card and system reliably.
The last time I upgraded a Dell power supply many, many years ago... Dell was using some proprietary connections that I had to get a specific power supply for. I would hope they don't do that anymore, but always double-check.
Edit: One feature my card has is that it takes up two slots, and draws and expels its own cooling air through the back of the case at that second slot. The GT 640 card you listed is more of a basic design with a heat sink that only dispels the heated air into the system itself. While The GT 640 uses less wattage, 65 vs. my 70 watts, another issue I'd consider is if your case's cooling is up for the job of that much more heat dumped internally. You may want to take a look at the back and front of your case to see if there is a place to put a second fan. Though a good power supply with a new fan may be all the extra cooling you need.