No chance of running F.E.A.R with a x300. Maybe with the lowest graphics and like 5FPS.
As I said Im not sure how well F.E.A.R runs with a 6600GT. I'd say it runs well if u just keep the fsaa at about x2 or x4 and dynamic lights at medium or low. The rest should be able to be maxed out, ofcourse ur other parts need to be good too.
But someone with a 6600GT should know better than I do.
You can run doom 3 on a 5700LE easly but not very good. Id say since your on a budget, but want to be able to get high resalutions and quality, id say a regular old 6800/non gt,ultra