You need to replace the "big 3", (board, CPU, & memory), to get anywhere with an effective "upgrade". Beyond the cost of the CPU you've mentioned, the board and memory could be had for a hundred bucks or so. I'm thinking a G41 chipset board (micro ATX size), would be ideal. That way, you don't need to buy a graphics card to get the system up and running at the beginning.
You could also consider a Core i3-5xx CPU, & H55 board. The prices are coming down, so "this year's model", so to speak, is right in the same price ballpark.
Depending on which parts you replace, and at which time, reinstalling the OS from your restore discs is not guaranteed. In different models of Emachine, you gfet different results. My T-5026 won't even boot from a regulation Windows disc, while other members have said you can reinstall Windows from the OEM discs , in the event of the board being changed. That's a crap shoot.
Your memory is DDR. DDR3 is now the spec, with entirely different socket an voltage requirements. Plus, it's at least 3 times as fast as the DDR400 you have now. (Average speed designation is DDR3-1333 to DDR3-1600).
As long as you choose a "Micro ATX" form factor board, you can use the case of the Emachines, at least temporarily. Keep in mind that the ventilation is poor in these cases. So, if you're looking to do high end gaming, then the case would have to go too.
![hellokitty[hk]](/community/data/avatars/m/169/169488.jpg?1355088591){kind=avatar}
