I still run on my "old" Ti4600. I've no intentions of upgrading for quite a while. I get very acceptable framerates in every game I play.
Mayhem, the 4600 engine outclasses and outperforms the 5200 in every way. The 5200 was marketed as the "value" line card. Of course you can find them for the prices you've listed as that is the market segment they are intended to be sold in. The only reason it acquired a higher major version number was because it was based on the next generation chipset, which happened to introduce native dx9 support. You want to compare the nVidia cards, then do the 4600 justice and compare it to the card that replaced it: the 5900/5950 series.