Well i would be happy to help you, i just dont know the prices where you live. plus the curency between £ and USD ($). so i will recommend some cards that i think you will like, but before i do so will you please list just a few of the games you are looking to play so i can help you the best i can.
Depending on who makes the card it may still be under warranty. XFX and EVGA are lifetime warranties so they would repair or replace your card. I believe if they can not repair it, they will send you a GTS250 in its place. I would try this FIRST.
Second, if you are upgrading, I would not recomment a 5770 or 4890 unless you were just looking for a replacement or a modereate upgrade from what you have. The 8800GTX is a pretty powerful card and can still hold it's own. If you want something that will give you a nice performance hike, I would look at a $5850. Obviously you are ok with spending a bit of money on cards as they 8800GTX hit the streak at $600 and still sold for over $300 the last time I saw it for sale.