I would put most of your limited resources toward a video card.
Regarding hard drives, how are your external HDD’s connected? If they are connected by eSata or USB 3.0, then they should be OK. If not, you can pick up an internal 1TB 7200 RPM drive for around $50.
The remainder should go toward a new card which should not only improve preview performance, but rendering times as well. What seems most often recommended is an AMD R9 card–ideally an R9-390, which unfortunately may be beyond your budget and also may require a new power supply. You might try to find an R9-290 (last year’s model) on e-bay or if that’s still too much, an R9-280 or even 270 series. Whichever you can afford, you should still see a vast improvement over your current Nvidia card.
Since it’s just one last project, you might also consider simply using what you have. Use of proxies for under-powered systems can ease the editing burden. Once happy, you can then swap back to the original files and let it render overnight or even for a few days.
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