Bootloaders are usually flashed in any one-click Odin package. If they are not, it will be clearly labeled as not containing them. They get flashed right along with the rest of the ROM, so nothing special to worry about. And yes, you would use the regular one-click eclair package to return to stock, and that will also replace your bootloaders.
Today, it is generally accepted as safe under all circumstance for a Captivate to be on GB Captivate bootloaders.
The best way to get there, IMO, is here-
[ROM][2.3.4][Gingerbread KH3]♦ Pinnacle 1.3 ♦ 09/24 - xda-developers
Download the 1.2 ROM and the 1.3 update. Below the specs, under installation instructions, is another file that includes the bootloaders, a special kernel, Odin, and a readme. This should get you where you want to be. You do need to be proficient at getting into Download mode, and not interrupting it... especially during the bootloader bit, which is first.
You do not need to change bootloaders when trying an i9000 ROM. You can probably get away with never changing them again.
Touchwiz 4 uses too much memory for our Captivate to run stable. The ROM I linked to above is SGS 2 themed by default, and tried to use TW4. However, our device does not have the RAM available that the SGS 2 does, and despite much tweaking and trying, it will not be stable. The preferred method is now a tweaked TW3 to look like 4, or any other launcher you prefer. The ROM I linked gives you a few choices to try.
I think Pinnacle or Fusion VI (both KH3) are the best choices right now. There has been three source drops since, and not enough time for any of them to be on the same level as KH3. Until a newer build stabilizes, the devs are working on a moving target.
One tip I recommend once you get updated. Either unmount, or remove, your EXT SD card before restoring from Titanium Backup. GB might otherwise install apps to your SD when it shouldn't- This causes performance issues on launchers and other apps, widgets wont work, etc. So far as I know, this is still a bug on the GB leaks. Be mindful too when installing apps that they do not go to the SD card, or if you do want them to install there, watch for problems and move it to the phone if you have any. I removed my SD months ago, so I do not have to worry about it

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