Same deal. The 550 ti is slightly better than the 6750, but the 6850 is a big jump up. 550 ti should be fine for the Pavilion, but not for 1080p.
Edit:
http://www.ebuyer.com/391772-xfx-hd-...e-hd-685x-zcfc
That's the cheapest of the cheap for 6850s. Your mileage may very vary.