[Build Help] Want to replace old ATI HD 4800 w. 6800/7700. Good idea (PC spec inside)?

Hey :) I've been thinking about updating ye olde rig for a long time but, honestly, I don't have a lot of money to spare... I recently went all out and bought Funcom's "The Secret World" and am enjoying it immensely - looks good (for an MMO) but as with Bioware's "The Old Republic" I've got a few lag and graphic issues. I think my graphics card might be the bottleneck and I'm thinking about replacing it with something at around ~150Euros.

My hardware: Intel Core i5 2500K on Gigabite P67A-D3-B3 | 8 gig of Corsair PC3-10700H (667MHZ) DDR3 RAM | ATI Radeon HD 4800 1GiG 870 Mhz / 1050 Mhz

What I am thinking about buying, after checking the "Best Graphics Cards For The Money: July 2012" guide:

GIGABYTE ATI Radeon HD6850 1GB DDR5 2DVI/HDMI/DisplayPort PCI-Express Video Card GV-R685OC-1GD

http://www.amazon.com/dp/B004F6GJAU/ref=asc_df_B004F6GJAU2088633?smid=ATVPDKIKX0DER&tag=pg-469-34-20&linkCode=asn&creative=395097&creativeASIN=B004F6GJAU

Review: Sapphire HD 7770 Vapor-X OC

http://hexus.net/tech/reviews/graphics/40701-sapphire-hd-7770-vapor-x-oc-edition/

What do you think? Would this be a reasonable upgrade or is the performance gain going to be negligible? Should I go with ATI again or switch to NVIDIA?