Depends on budget, really. You could start at a GTS450 / HD5750, and anything from there upwards would be a noticable upgrade over a 9600GT. At the minute there isn't really
a sweet spot
tbh: the bang for buck is very similar across the GTX460 (768MB and 1GB), HD6850 and HD6870, depending on whether you want to spend £130 or £180.
The GTS450 / HD57x0 generally aren't quite as good in terms of bang for buck, but the exception to that (thanks to a good price!) is
this Palit Sonic edition GTS450 which will perform to about the same level as a HD5770, but is only £95. So if you want to keep the budget tight I'd go for that: otherwise a GTX460 or HD68x0 is the order of the day