With that budget, you won't be getting anything huge improvement in performance in comparison with what you have now. You could always overclock, but I still don't think it would be worth the upgrade.
You also don't mention what exactly you'll be doing with this computer, and why you're not happy with your current one. Unless I'm assuming incorrectly and this computer is for someone else.
You could easily upgrade you mobo, proc, and video card for less than $700 to a pretty good s939 setup. If you're looking to completely rebuild, though, I'm not sure
Are you starting over???? It would be hard to make under 700. For a really nice system you might look at the shuttle. AGP=sn95G5 with a 6600GT and 1 G value ram. s939 3500+ would put you right at 900. the new sn25P5 with a similar set would be about 100 more but with PCIe and more watts in the psu. These are really nice systems,very complete and easy to work with.Plus compact. I have a G5,3500+ and 6600GT and at 1024x768 it games like crazy.
Just upgrade your motherboard and CPU, keep everything else, with the exception of your power supply. We need to know what kind of power supply you have.
Leaves you $30 for shipping. I know you live in Mexico and Newegg can't deliver to you, I just use them for the price ballpark. You should be able to find these parts around the same price somewhere else.