The GTX285 and GT640 appear to score very closely in overall performance, the GTX285 appears to be slightly better although a factory-overclock on either of those cards would likely put it ahead of the other. Your FX-9590 proc and CFVZ mobo have no integrated graphics.
Yes, it can be done. Just plug your GTX285 in the primary GPU slot and the GT640 into any other GPU slot, do not link them with an SLI bridge, tweak your BIOS PCIe lane settings if needed to POST and boot, and configure a PhysX card in the GeForce options.
But you have the fastest-clocked AMD CPU on market (indeed, even faster than Intel's fastest processor offering) and the mightiest motherboard it could run on. I really think you'd do a whole lot better investing in a better GPU card than trying to get your old ones working together, your elderly GPUs are a huge gaming performance bottleneck. I see GTX660Ti/GTX750 and HD7970/R9-270X cards go for less than $50 on my local craigslist. And, even when optimally configured, you'll only enjoy noticeable PhysX gains on a handful of older titles.
"All opinions are not equal. Some are a very great deal more robust, sophisticated and well supported in logic and argument than others." - Douglas Adams
[/Korth]