You 'may' be able to squeeze it in anyway.
The GT640 uses between 50-65 Watts.
The processor in your computer uses 65 Watts, and hard-drives typically consume between 6 and 10 Watts.
Considering the computer is not a performance driven build, the motherboard should be sitting at no more than 50 Watts.
Once you account for the optical drive, a couple sticks or RAM and 2-3 case fans, you are pretty close to your limit.
Two things help you out in this case though.
1) The wattage for a Power Supply is usually under-rated, and is difficult to judge due to the fact that the wattage is split up between the +12, +5, +3.3, -5, and -12 Volt outputs. If the
2) The wattage quoted for the components are at 100% load which would realistically only be seen during extended stress tests of the machine.
Remember that for the machine to actually reach this limit, you would have to be reading/writing to the hard-drive excessively while the CPU is bogged down running at 100% while you are playing a game that is maxing out your Graphics card.
Not all computer power supplies are created equally though either... The harddrives for example rely heavily on the 5V rail on a computer power supply, while video cards rely on both 5V and 12V, with many higher end card requiring an extra 12V connection. So simply adding Watts together will never give you an ideal answer because it's entirely possible to overload the 5V output on the power supply but still have plenty of room on the 12V output when your total wattage is less than the PSU. It's a flawed system, but it's 'easy' for quick simple answers... Unfortunately, you need the details so it becomes a little more complex.
Personally, if I were in your shoes, I would see if my friends had any old PCI-Express cards that I could try/have for free... It's old and they don't need it right?
If that didn't work, I'd research the requirements to swap out the Power supply, then risk running a 640. I would be confident the 640 would not push the power supply over the edge, but it's too close for me to give you that guarantee. I had read that your PC came with a 300W supply, but perhaps that was a recent change.
(And FYI... I've done a swap from an old PSU to a new one for an HP... I hated it because they didn't use a standard header for the PSU...
The little black connector on the left of the board (2 rows of pins) is what HP used... so I had to cut off the connector on the original power supply and solder the wired to a female connector. I now have a handy home-made adaptor that lets me use any power supply with a standard plug on this annoying custom motherboard in the HP. Take pics of yours, and we can see if it can be done.)