One problem with doing that. Your ram is going to slow down to ddr333, which is going to hurt performance. The FSB will also slow down to 533Mhz which is going to REALLY hurt performance. It would be a smarter idea to keep the 7900GT for now, and pick up the following parts when you have the money:
CPU: Intel Core 2 Duo E6300
Mobo: Gigabyte GA=965P-DS3
RAM: OCZ Gold 2GB (2 x 1GB) DDR2-800
Power Supply: Antec Truepower 480W
If this simply isn't an option, your best bet would be to go for This board
The board that you pointed out is not compatible with Pentium 4s. It is fore Pentium M processors.