The performance difference should be nothing. Usually the difference is in bundle or warranty. Who do I trust? Asus, MSI, Uncle Jim down the street, and then Sparkle. I have no idea what the names stand for, probably nothing.
Overall, the HD 5770 is a little more powerful than the GTX 450 and it also uses a little less power than the GTX 450; 77w vs 84w under typical gaming conditions. However, the GTX 450 uses a little less power than the HD 5770 when idle and when playing back HD video.
I got mine at microcenter, you pay more up front, but if you take a chance with the rebate, it's supposed to bring the card to 85. I came from a 9600gt also, mine didn't die, sold it. But the 450 is great for it's price.
As far as brands, MSI is good, Sparkle not sure on. ASUS is supposed to be good.
On the Galaxy brand, I've had good luck. My 9600gt was a factory overclocked card. I had that for about 2-3 years, then sold it to my buddy who built a system, but didn't get his video card yet. He was puttering along on a gt220, so I sold it to him as a holdover until he gets a better card, but it's still running fine for him also.