That combination will be fine IMHO. I would still check what PSU you have ATM though!
Make sure you activate the free postage and look at the following web-pages for special offers:
http://www.scan.co.uk/todayonly/inde..._icmp=AkaIX3H4
http://www.scan.co.uk/shops/homepage..._icmp=7Uz238zS
Edit!!
The PSU is on offer. There is also the 1GB EVGA GTS 450 SC on offer for around £84. It might be faster than an HD5770 1GB but has a longer PCB and consumes more power although the XFX 450W PSU will be fine.
The 1GB EVGA GTS 450 SC will be around 5% slower than the overclocked Asus and EVGA GTS450 cards in this review from my estimates:
http://www.hardwarecanucks.com/forum...-review-7.html
It still looks like the HD5770 1GB is better value IMHO.