A graphic card running between 45C & 57C is quite normal and nothing to worry about.
Third party cooling will help and will lower temps as well.
For £130.00 to buy a single card I'd be tempted to say stay as you are with the two ATI HD3650's in Crossfire, add a pair of VGA coolers and overclock them a little using Riva Tuner.
You won't notice a significant performance increase between a single card for £130.00 and what you have now so imo you may as well stay as you are until you can afford a more major grafix upgrade.
Just my opinion
If I were going to spend that budget on a single card, I'd get
This One