Thanks everyone... finally got the explosion working.... so much nicer! 
im happy that you figured out the solution but can you tell me what it was! thanks
1. I put the Galaga sample in my C:\MAME\samples directory without unzipping the files.
2. I was getting some sort of error message about the '54xx.bin' file, so I loaded new '51xx.bin' and '54xx.bin' files.
When I ran it again... KABOOM!

So, I'm not sure if it was that I left the samples zipped, or if it was that I got better files.