Heres a link to GPU listings:
Graphics Card Hierarchy Chart - Review Tom's Hardware : Best Graphics Cards For The Money: Jan. '09
You would jump 9 tiers by getting the HD4650 and heres what they have to say:
"I don’t recommend upgrading your graphics card unless the replacement card is at least three tiers higher. Otherwise, the upgrade is somewhat parallel, and you may not notice a worthwhile difference in performance."
So you should see a noteable increase in performance however the HD4670 are really cheap just now so if possible id advise that over the HD4650. The price difference between the two shouldnt be that much.