What exactly is the '450W' PSU? AFAIK no one sells a decent 450W PSU in Sri Lanka. Everything decent I've seen is 500W+ with the exception of the Axle 400W (which is pretty good). If it's a generic 450W that came with the PC, it's probably got a true output of around 220W. Check on that first. Chances are you're going to need a new PSU as well.
Also, Axle is a great brand for the price. And knowing the agents, you will get the best customer service and warranty you can find in the country, period.
About the card, I run a Axle HD5770 which is almost identical to a GTX550Ti, and it kinda reached it's limit with the BF3 Beta at 1280x1024. You might want to go for the GTX460 or HD6850 which costs the same.