tell me in all honesty as the old pc im on uses a gforce 8400gs from 2007 on a single core amd 2.2
moving upto a quad 965 would i notice difference between 560/570 and is the price jump from aprox 140/180 pounds GBP worth the jump upto 240/260 pounds GBP for a 570
your old PC is ancient so anything modern would be a noticeable difference. it really depends on your monitor resolution. i'm guessing your 23" is 1080p so yes you would notice the difference between a GTX 560ti and 570 if you want to run games max. if you are willing to adjust some settings to high from very high, the GTX560 / AMD 6870 is fine.
but ude go the extra mile for 570 right ?
if the extra cost won't affect you then the GTX570 would give the best performance.