I guess what I really need to know now then is there a better choice than the GF6600 GT thats still a relative bargain and does not require a motherboard upgrade to utilize PCI-e?
Honestly, seeing the way FEAR even makes my machine weep, I think there really is NO budget upgrade that you can make that will give you great framerates.
The 6600GT is probably your cheapest and best option, but don't plug it in there expecting +100% performance.
Fear is a brutal resource hog, and not at all indicative of any other game currently out. I think if you get a 6600GT, you will be VERY happy with it for all your other games for another year to come, when you can hopefully do a full upgrade.
You's picks your poison.