I got a new 1080p monitor from 720p and noticed a lot more slowdowns. I also monitor my GPU usage when playing AION and it hits 100 pretty frequently so I thought I should update my card.
I have a Radeon HD4850 PCI-Express Video Card 1GB DDR3 Ram which pretty much perform exactly like a 4770 (I have a 4770 and switched it out with this card because BF3 with textures on high need > 512 MB of ram)
I made the jump to a 6950, I would say check ebay. Never had a problem buying through that site, and I have built 3 computers using Ebay. I had a 4850 and I loved that card but the 6950 just eats it up comparatively but I also paid 250 for it. A 6870 might be a great choice or you could hunt for a 4850 x2 since its got the same performance as a 6950 but fora whole lot less