You're really going to need to fill us in with some details, but one thing comes to mind right away.
You do have 128 MB of RAM (at least, and hopefully you have 512 or more), right?
You might want to try using the OS X installer to erase the hard drive you want to install on first. Obviously, you'll need to back up whatever important documents and things you want to save. Honestly, that's just a random guess (as mentioned, some more info would really help), but it's also sound advice for a major upgrade like the one you're attempting to undertake. You'll get better performance from your system if you do a clean install.