Preliminarily on the specs you have given
There is not much difference between the CPU's, the 6420 is slightly better on paper, but only a small bit, certainly not enough to warrant a swap.
Your 5450 Gfx card is far better than the 7300.
The only extra to what you have is the extra ram and Windows 7 (depending on which version)
Personally I would not bother, but it depends on what you want the swap for.
BlueLou is right though, the amount of garbage installed can make a huge difference to your speeds.
I would also recommend CCleaner, Autoruns and a decent browser with an ad blocker.
If you need any help let me know