A $25 price difference after rebate between the two cards is not considered "significant".
Without a doubt I would buy the HD 4670.
Not considered "significant" by who? In this price range it is certainly quite significant and before rebates the HD4670 costs almost 40% more.
Furthermore on his processor even the HD4650 is overkill, the HD4670 is pointless.
I have an ageing AGP system. Back in 2008 I managed to get an HD3850 for £100 from Sapphire. Much better performance than the 3650 and certainly good value for money. I fail to see what a 4xxx card would offer that this one couldn't, except HD audio passthrough which isn't even supported for AGP interface.
If you can get this card now, its still a very good DirectX 10 card. Check on Toms Hardware for the ranking as well - still pretty respectable!!!