I want to buy new graphics card GTS 450, I already have a GT 240 card with no SLI.
My doubt is can I use dedicated physx processing from GT 240 and graphics processing from GTS 450.
If I can use that feature recommend me the supported mothers boards, currently I have a single PCI-e slot.
A 650i is a poor choice and back then many weren't that good so the same still applies today. If you are into Nvidia enough to consider such a board then a 780i/790i is the sweet spot as they are covered still under warranty but EVGA requires proof of purchase for any support. XFX however you buy a dead one off eBay and RMA that for a working copy as I have done but beyond that stay with Intel chipset and nothing else. The 750i isn't much better but all the same as a 650i with a N200 pci-e 2.0 switch.
Any model that has two or more PCIe slots will work for you if you just want to run two different Nvidia cards, it does not need to be SLi certified it just needs to support your CPU and have two slots.