Right, as Nickjf20 has said, it could be fine but as I don't know, I don't want to recommend testing as it's not my system that's going to be testing it. Personally I would try it but I take risks.
https://www.overclockers.co.uk/showproduct.php?prodid=GX-248-MS&groupid=701&catid=1914&subcat=1854
That is £10 less but the fans on the Asus card you listed seem better for only £10 more so I'd go with that if you choose to get a 750Ti. Either card is better than your old card and will not stress the PSU out.
I'm assuming your old card was a HD 4850, if so I think the 5770 is comparable. If I'm right this is the boost you'll get with less of a power draw.
http://www.anandtech.com/bench/product/1130?vs=1078
https://www.overclockers.co.uk/showproduct.php?prodid=GX-248-MS&groupid=701&catid=1914&subcat=1854
That is £10 less but the fans on the Asus card you listed seem better for only £10 more so I'd go with that if you choose to get a 750Ti. Either card is better than your old card and will not stress the PSU out.
I'm assuming your old card was a HD 4850, if so I think the 5770 is comparable. If I'm right this is the boost you'll get with less of a power draw.
http://www.anandtech.com/bench/product/1130?vs=1078