This guy meant,like that you won't play crysis on normal...,probably on low.(depends on your monitor resolution!)

You can still buy something like a radeon 4800 series,will give you better performance of course...probably more than double.
Well,a SSD would be a better idea i think,for speed.Would not boost games but it would be a more interresting approach to boost the whole system and OS.
But for performance in games,any GPU will be great of course,i know

and you want performance in games,so...
I don't know where you can buy old cards tho,i mean, new... You can buy used cards but you don't know how much time they will last.
You could buy one to one of the guys here maybe:) would be a good idea,i'm quite certain that some guys here have a good old card that is still fresh :D,and would be cheaper.
And how old is your PSU?