Have you tried the Apple hardware test? You'll need your original OS X DVD's that came with your system.
Did you try to boot to your OS X (original that came with your system or retail version) DVD by inserting the 1st DVD and holding the C key down? If you can do that, bring up Disk Utility from Utilities on the menu bar and repair your hard drive. Repair permissions while you're there too. If that completes without error, reboot the system.
See this Apple note to reseat your memory. If you have 2x1GB cards of memory, try booting with one then the other, to see if one memory card is bad.
If a safe boot fails, you can't boot to an OS X DVD or run the Apple hardware test, and reesating memory doesn't change anything, then I would take it in to the Apple store for service.