I do not see why you would want to upgrade this PC.
It's very good as it is.
If you want it to be more responsive, buy a SSD.
If you want better performance in general applications, overclock the CPU.
If you want better gaming performance, grab a new graphics card or SLI your current one.
But really, apart from the SSD, the other upgrades make no sense at least to me.
I'm sure you can play every game on high settings smoothly and max out all not so demanding games.