Should I buy a NVIDIA Geforce GT 640 ? If yes, would it fit in my PC? till now I have a Nvidia Geforce 8400 (which unfortunately doesn’t support CUDA 1.3+ ); is it the same slot ?
and i just have vista 32-bit
I have the 640, the zotak fanned one, its quite tiny and I think its a single slot depth so not overly large at all.
Speed wise it is quite slower than the 5* cards but still a lot faster than my Intel quad core, although I think that its 2 gig memory will be the kicker tho when I start to do some more adventurous stuff.
The good thing is it doesn’t need any additional power connectors as its power requirements are quite low so can take it from the pci/motherboard.
I was forced into upgrading to it as amd dropped support from my previous card 6300D or some such and as I am “between jobs” it was the only one I could afford, but it does what it says on the box 
problem solved; i’m not going to buy it, because my computer is too old (only 288 Watt max. and an old Phenom CPU). I think, I have to continue using my iMac’s CPU 
it is still pretty fast
In a word, no
Updating only one hard ware will not help , you should have all the hardware equal to make your computer good and fast. If the problem is solved than I think that you should edit it and change the title to solved. 
Just a suggestion