Bad things will happen. Do not take the OTA update. I'm honestly surprised you're getting the prompt with a custom ROM.
You have two options.
1) Wait until Eclipse comes out with the 2.3.4 update integrated.
2) do a factory reset, wipe cache and data, SBF back to a stock image, and then take the update. This is by far more tedious as you will either have to back up your app data with TiBu, or lose that app data. If you do this, make sure your phone is fully charged before you start any of it.
I nearly forgot option C. Don't upgrade, download an app to block OTA updates, and keep on truckin'
Your call.