IIRC the 6450 has very roughly half the processing power of a 4670 -- 6450 has half the shaders, clocks are a bit higher, but memory bandwith is lower. Tom's "Hierarchy of Graphics Cards" puts the 4670 ~5 steps above the 6450.
The 6450 pretty much splits the difference between the older *4** parts and the older *6** parts, if that helps.
EDIT -- Yikes, I see how you ended up picking that card. The next fastest one is more than twice the price...
Anyways, check the performance numbers... the 6450 will probably be slower than your 4670, question is whether it is too much slower. I would look for a 5550/5570 or 6550/6570 if you can find one at a good price.