On second look the GTX 560 Ti 448 does not use quite as much power as a GTX 570 but it does use more than a GTX 560 Ti.
You might want to go with 750 watts just to be sure. Guu3d recommends 750.http://www.guru3d.com/articles_pages/geforce_gtx_560_ti_448_core_review,7.html
Thanks to both of you for your answers. Anort3, you were right, for some reason I thought my 650 had enough of the PCI-E connectors with it but it didn't. I'll probably end up just not doing SLI then, thought about it because there was a nice 560 on eBay for a decent price but if I have to upgrade PSU in the process it would start getting to the range where I would probably be better off just picking up a 600 series card and forgetting about SLI altogether.