Unfortunately I don't know of any benchmarks for Xilissoft and the only one for editing is for Adobe CS5 which compares performance differences running with and without CUDA on CPUs with different number of coreshttp://www.tomshardware.com/reviews/adobe-cs5-cuda-64-bit,2770.html
The above article used a GTX 480 but I have seen other articles about CUDA showing substantial time savings just with GT 240. I have no idea how it scales.
Frankly I am surprsied that the nVidia site does not have more on this - or at least I did not find it.
If you decide to upgrade the PSU, here is a good bargain right now:
Antec Hi Current Gamer Series 620w PSU - 80% Bronze certifed - $50 after rebate with free shippinghttp://www.newegg.com/Product/Product.aspx?Item=N82E16817371048