I can't get my Samung Captivate to boot anymore, please help!?

Ok so I feel like I'm pretty screwed on this one. About a week ago, I rooted my phone and installed the rom cognition with ROM Manager so that I could use flash. Later, my phone stopped detecting the screen orientation so I decided to try a different rom. I had the most issues with the last one because it was for the i9000 instead of the I897. Finally, I decided just to restore the stock rom from the backup I made with ROM Manager. Now for any backup I choose from the clockworkmod recovery menu, I get an error saying "md5 mismatch" and any time I turn on my phone it shows the at&t logo and then the Galaxy S GT-i9000 logo and just stays on that logo. I can't boot into Download mode because none of the key combinations work except the one to get into recovery and when I use adb it says "device not found". Is there anyway I'll ever get this phone back to normal without having to buy a JIG? Sorry if I mispelled any words or left out any details, it's just that this phone was a Christmas present and I'd hate to have destroyed it after only having it for two weeks.