The x4 845 is newer in theory, but based on older architecture. Clocked higher
It's by no means a powerhouse.
The i5-750 is older (launched in mid 2009!) and clocked lower, but it's probably a little more 'fit for purpose'.
Considering the $50 investment to buy the CPU, not to mention a new motherboard being required............I don't think there's a clear winner when all aspects are considered.
I'd stick with the x4 845 if you already have it & the appropriate motherboard.
If not, there are better options available.