Another intermediate option between the 550ti and the 6870, in terms of performance and price, is the Ati 6850. On amazon it is at about 115€ (+ shipping), and I think on pccomponentes it is more or less at the same price.
To give you an idea of the power of each one, you can take a look at this table from tomshardware that adds up the results of several tests that have been done on a lot of cards... it's not that it's the bible but it will guide you.