To a certain point, you're right. AGP is on it's way out, but there are still some VERY good AGP offerings on the market. I'm running a 6600 PCIX card right now and it screams compared to some other AGP cards in the same range. I'm also going to back up the statement that you have AGP. The FX5200 wasn't made with a PCIX version.
The 6600 is definately a good buy for the money... hell, I wish you had PCIX because I'd sell you mine. I just bought a 7800 GT PCIX. My 6600 was a cheap upgrade at the time I rebuilt my computer last christmas.
I'd personally stay away from ATI cards, but that's just me.