Quickie guide to installing ROMs.
1. Grab the desired ROM of choice. Cyanogenmod 10/Slimbean is a good start
2. Get the gapps for the ROM. http://goo.im/gapps/
3. Get cyanogenmod7 from http://download.cyanogenmod.org/?device=captivatemtd
4. Get CWM here: http://www.semaphore.gr/kernels/Semaphore_2.5.0c.tar
5. Heimdall here: http://www.glassechidna.com.au/products/heimdall/
. You don't need to install anything extra with 1.3.1
6. Copy the gapps, CM7 and your ROM of choice into the internal AND external SD card. Some rom comes with a kernel that don't play nice with either SD card, so both is better.
7. Extract Heimdall, go into the driver folder, open Zadig.exe
8. Pull the battery of the phone, put it back in. Hold Volume Up + Volume Down and plug in the USB cable
9. Once Windows detect your phone, click on option -> List all device. Look for Gadget Serial Control, Samsung Composite Device or similar. If it look like your mouse/keyboard/webcam/other internal component, don't touch it. Click on Replace Driver, and wait for it to install.
10. Go back into the heimdall folder, hold shift, right click the folder and select open command prompt here.
11. Using 7zip/WinRAR, copy the content of the Semaphore_2.5.0c.tar file to the heimdall folder.
12. On the Command prompt, type:
heimdall flash --kernel zImage
13. Once your phone reboots, it should bootloop. No problem. Pull the battery, put it back in, hold volume up + volume down and press power until it turns on. You may need to do this a few time. It should boot into recovery mode.
14. Select install zip file from SD card -> Choose from Sd card -> the Cyanogenmod 7 file. Confirm with yes
15. It should reboot automatically, and bootloop. Do the pull battery trick from 13 to get back into recovery mode. Do 14 again. This time it shouldn't bootloop.
16. Go back to the main menu - Wipe Data/Factory Reset -> Reboot
17. Wait for it to boot.
18. Go back into recovery mode, and Wipe data/factory reset again. Install the ROM of your choice, along with the gapps. Newer ROM should ask you to flash it again to confirm. Do it.
19. After flashing, wipe data/factory reset again, before rebooting