The 4670 like you had originally listed had 1gb of vram but was not overclocked and a 4670 can't use more than 512mb. The one I linked to will be noticeably faster.
As for the 4830 I liked to one on newegg that has an IceQ4 cooler that will keep it at lower temps and will allow for overclocking. It is about the same price as the one on Tigerdirect after the rebate.
Like I said earlier though the 4830 is a good amount faster but please note that the 4670 does not require an external power connector.