How much are you expecting to get for the used 7770? You can buy a new 7770 for 89.99 on newegg. Also what is price of the 650 TI Boost you are planning on buying?
If it will only cost you $20 I would say its worth it, but it might cost a bit more than $20.
Mine is the Vapor-X which is going for 119 new. http://www.newegg.com/Product/Product.aspx?Item=N82E16814102993 The 650 TI B's start off at 119, not counting rebates, so maybe im not taking into account the loss of value of my card but it seems like a good deal so far.
(Gameweasel quoting is not working, ugh)
Steam & XBL - xNo L1m1tZx, Origin - xNoL1m1tZx Currently playing BF3, Recon only, go ahead, cry some more.