keep the 4600+, keep your GTX since its really close to the 1800xt in performance (definatly NOT enough to justify $200 more) and atleast wait for the 1900/7900 to come out.
more then likely the 1800 will drop in price when the 1900 comes out, which should be this month. your more then likely gonna run into more problems trading off single core to dual core, than trading in a GTX for a 1800xt in the future. not to mention why do it at all since that GTX *will* last you atleast 2 years, and if thats not enough for you and you HAVE to play everything at 1600 4aa 8 af all the time, itll still last you till the G80/R600 come out. my advice would be wait a little while.