your GPU is a low end. if you want to play games with higher resolution/detail, u need a more powerful gpu. And if you planning to upgrade, also replace the PSU with a 450w++ PSU. first gen phenom was a power hog so i dont think a phenom and a mid-high end gpu will be handled properly with ur PSU
Ya to be honest personally I wouldn't upgrade what you have now the 5450 is a pretty low end card that is really better for HD movies then gaming the phenom isn't to bad but you wont be able to upgrade your GPU with out a new PSU also with DDR2 you are going to see slow down there as well .
I'm in complete Agreement with JIT any piece of hard ware that you upgrade and install now is going to be so badly bottle necked bye the CPU and ram that you will see a very very minor improvement not worth any investment.