All the points above are good. What motherboard do you have and how much overclocking, if any, will it allow?
If you can't overclock, even if you plug in a 9800 GT, you'll get improved performance but you won't see the full performance of the new GPU until you can increase the fsb and the frequency of your CPU.
The HD4670 will be about 50% more powerful than the 8600 GTS.
The one you are looking at is a lot more expensive than necessary though; http://www.newegg.ca/Product/Product.aspx?Item=N82E1681...
The difference in memory size is entirely meaningless as the HD4670 cannot properly use more than 512mb.
The % performance increase from 8600GT => HD 4670 will be significant.
I hate it when people miss the difference between 8600GT and GTS - there is a huge difference in performance. But never the less - nice graphs; they are very self-explanatory.
Neitherway, I'm neither going with a 4670 or a 4770 - depending on prices. I will also be getting a new PSU so I can use my old PSU to build a media center - my older PC is P3 with 4GB DDR2 and a 2400 HD Radeon AGP with a fried PSU - it's outdated but I'm sure it can handle being connected to my 50 inch TV to watch shows in .AVI format.
It was probably just a typo, the graphs he provided did include the GTS and not the GT. Considering you were talking about the "HIS Radeon HD 3760 IceQ " in your first post there's a saying about glass houses you should read up on
The test system for the charts above used an an e8400 @ 3.6 GHz. Back to my point, his current CPU @ 2.2 Ghz is more of the problem than his old videocard and he should look at replacing both and not just one component.