If you go for that GTX 760, you'd only be throwing away good money after bad, in your instance (as cc_merchant has stated). I would only get that GPU if you're planning to upgrade the CPU and/or motherboard, as well as the amount of RAM and an additional number of disks, in the near future. If on the other hand you'll be stuck with that dual-core i3 for years to come, don't waste your money on anything higher than a 1GB GTX 650 since the i3 CPU doesn't handle the editing of any video at any resolution above 480i SD very well (I was going to say 320x240, but almost all 320x240 video is encoded in distribution codecs that are unsuitable for editing). The 2GB version of the GTX 650 is overkill for that present i3 setup, in this case, since the CPU just can't handle any content that requires more than 1GB of VRAM well.
As for the GT 640, be aware that there are two different versions of the GT 640: Most top-tier graphics card brands still sell the original version of the GT 640, based on a GK107 GPU and using 128-bit DDR3 VRAM that has a throughput of only 28.5 GB/s. A few lesser-known graphics card brands have started to sell a newer version of the GT 640, based on a GK208 GPU and 64-bit GDDR5 VRAM that has a throughput of 40.1 GB/s. But given the relatively low clock speed of that i3-3220T and the fact that it neither has a Turbo Boost feature nor can be overclocked at all, even the original GT 640 may suffice for many uses for that PC.