The 5750 will be faster, cooler, and use less energy. And PCI Express 2.1 is backwards compatible with version 1.0 and 2.0 so it will work fine. My brother in law is running a PCI Express 2.1 HD 5770 in his old Nforce motherboard with a PCI Express Rev. 1.0 slot. If you can spare the extra 5 bucks though, I'd seriously consider stepping up to the 1GB version. Games require more memory every day.
thanks for the gread advice. I have a gigabyte GA-EP45-UD3R. would that work with the 5750?