There are still a good bit of boards around that can run the older 775 Pentium 4's. Note that once you change the motherboard a fresh copy of windows has to be installed also if your system is an OEM like Dell, Gateway, HP to name a few the copy of windows that came with the computer will not work since it is tied to that original motherboard.
GIGABYTE GA-G41M-ES2H $49.99http://www.newegg.com/Product/Product.aspx?Item=N82E16813128396
This is the cheapest motherboard I could find that will work with the Pentium D 830 the problem with the old Pentium D's was that they were very power hungry and the cheaper boards then this one could not handle CPU's over 105watts the Pentium D 830 is 130 watt CPU.
I would not suggest really putting too much into that old system since you could get something 10 times as fast for around $100-$150 for motherboard and CPU.
ASRock K10N78 AM2+/AM2 http://www.newegg.com/Product/Product.aspx?Item=N82E16813157159
AMD Athlon II X3 445http://www.newegg.com/Product/Product.aspx?Item=N82E16819103872
Its just something to think about since its so not worth putting the money into that dead EOL system.