Yes, you can use the 800 MHz board in place of your 700 MHz.
Did you not try the source I pointed you to?
* [https://www.macpartsdepot.com/820-1257-a-820-1381-661-2589-logic-board-700-mhz-for-imac-g4-15-m8672ll-m7677ll-m6498.html|15" iMac logic board 700 MHz, 820-1257-A, Apple P/N 661-2589]
* [https://www.macpartsdepot.com/820-1398-a-661-2841-logic-board-800-mhz-for-imac-g4-15-inch-m8535ll-m6498.html|15" iMac logic board 800 MHz, ''820-1398-A, Apple P/N'' 661-2841]
I’m sure you can also find the part searching Google by the part numbers.