Assuming your stock CPU is a Core i5 650, then the Xeon X3450 is a reasonable upgrade, even though it's a year older and has lower clock speeds. The extra two cores will be of more use today. The Xeon's memory controller is integrated in the chip, unlike the i5's which is a separate die underneath the heat
spreader (I.e. there are two dies in the CPU), so that will help a little bit too. Don't expect massive improvements though.
Downsides to using the Xeon? It's a 45 nm die, whereas the 650 is 32 nm, and coupled with the extra cores, it will use quite a bit more power and therefore generate more heat. With all four cores loaded up, the Xeon will also run a fair bit slower than the i5.
The most important difference is the fact that the Xeon doesn't have an integrated graphics processor, so if your PC doesn't have a discrete graphics card, you won't be able to use that CPU.