it's not just the 450 part, it has to have enough amps on the 12volt side and not just the 3.3 and 5 volt side. A lot of older power supplies had a lot of power on the 3 and 5 volt side and less on the 12 volt. Problem is now that new stuff use more power on the 12 volt side and les on the 3/5 volt side. Newer cards like the geforces pull it all off the 12 volt side. You might have 450 ( peak) but it might only be 350 or so continuous, plus it might be on the 3/5 volt rail which does you no good.
You need to post your model number and such. A good place to go would be jonnyguru.com, their main focus is power supplies and people over there know a lot about them and what is inside. You need to give us the model number and a picture of the sticker though to be able to tell for sure
Hmmm...it might work, but it could also die, depends on the quality of the build and if your gonna overclock, but I think it is pushing it on a GTX. I am no expert though, so I would go to that site and ask the people that actually test power supplies. A lot of inexpensive supplies fail WAAAY before their rating around 50% load on some, so it is best to see if anyone over there would know.
Ive used a 420 watt with 18 amps per rail, a bit better quality than yours, with a G80 GTS, but I felt I was coming close to maxxing it out. I wouldnt do it, not sure about aopen, and its no better than my old enermax