The HD 4670 will be faster than the X1900XT by about 10% - 15% based on research I've done back when the HD 4670 was released. I would recommend you look at the HD 4830 / HD 4770 / HD 4850 (in order of increasing performance).
Spending about $60 on the HD 4670 for a minor increase is not worth the money. I too am looking to upgrade my aging X1900XT 512MB, and I am looking at the HD 4770 as an interim upgrade for when an "affordable" but powerful DX11 card comes out.
The HD 4770 is a $99 main stream card that will dominate the X1900XT, yet consume less than half the power of the X1900XT.
I am waiting for XFX's version that will exhaust hot air out the back of the card, but the it is not expected to be released until the end of May or beginning of June.