No question -- GF6600 GT.
do NOT buy a 6600GT expecting great performance out of FEAR
Well that clears things up!
Seriously though, I appreciate the suggestions. 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? I'm just looking for the best (i.e. inexpensive) solution that will hold me over until I can afford to do a proper upgrade.