As always (I must sound like a broken record), what are your applications?
Your existing system will beat the new one at encoding, number-crunshing, and probably C/C++ compilation.
The AMD will blast the Intel for things like python, php and, at least in 64-bit OSes, with mysql, and most games.
You get a Via chipset which will be a huge setback from the i875. The i875 mainboard is much better, especially with its CSA LAN if you network locally. If you run anything but 32-bit Windows you'll might have drivers issues, and lots, while the i875 is roblem-free.
You are also making a very expensive choice with the Socket 939 setup. You see the 3500+ is only 2.2 GHz. A Socket 754 Newcastle is 2.4 GHz and $220, and it will be faster for almost all applications. Your 3500+ doesn't even has the big cache, and even if it did few things beat clockspeed once you have a big enough (512 KB) cache.