I would think that for a 1080p monitor, 1.25gb should be enough. After all, it was a great card last year...
Out of the two, go for the 7850 if you can. It's not like the 560Ti 448 is a bad card, though
Just a little update guys. After inspecting my old power supply and card it seems I only have an 6 pin connector to power this card with. Do any modern cards use a 6 pin?
+1 for the 650ti. I have a mate who's got one and it handles everything he throws at it pretty well. The most demanding thing he runs is Skyrim @ 1080p and it plays well above 30fps on good settings (I'm not sure the exact settings). Not bad for under £150 really.
If you didn't get one yet, I just got a Sapphire 7850 2GB OC for 150, runs nice and cool, quiet, only one 6 pin connector needed. Not much more you could ask for at this sort of money.