p.s. your mac is not G5, it is Mac Pro 5.1
Actually it's a cMP (Classic Mac Pro) 5,1. A replacement card really depends on what you want to do with it. My 5,1 is now on it's third GPU. I've replaced it over the years to keep up with the times, reduce power draw while still having a powerful card. My current GPU is a MVC flashed GTX 1080. That may be out of your price range.
Many folks here, keep touting the GTX 680, because it was the last third party Mac Card. But, it's an old, outdated card IMHO. Also it's available in 2GB and 4GB versions. The Mac Edition GTX 680 was manufactured by EVGA and is a 2GB card. A genuine EVGA Mac Edition is quite expensive because they are so rare. Flashed ones are really available on eBay.
Non Apple GTX 5870s that have been flashed are available from a number of sellers on eBay for about the same price as the GTX 680.
Now, if your were happy with your ATI HD 5770, and want nothing more powerful, my recommendation would be this card:
http://www.macvidcards.com/store/p15/Nvidia_GTX_570_GB_1.25_GB_or_2.5_GB.html''
It's also outdated, but reasonably priced and you will get a boot screen, and from what I see will work for you
The seller is reputable. This was the first card I had in my 5,1 and it performed well at the time. I still have mine sitting in a box.
I see you're in London. MVC has a London affiliate. Contact MVC and ask him how to get in touch.
Good luck with your choice.
Lou