MacBook :: Blank Boot - Potential Logic Board Fault?

After using a PC laptop for six months or so (and it really messing with my eyesight) I thought I'd try to revive my old macbook. Basically 70/80% of the time it won't turn on. There are these patterns of faulty behaviour which I hope you guys could diagnose:

1. I'll press the power button, I hear the disk noise, and the screen is black (i.e. not a backlight problem). The chime won't sound, I can just hear the fan and the hard drive whirring. The only way to get out of this is to power it off via the power button.

2. Sometimes after it's started up successfully and I move the Macbook (say, from a desk to my lap) the same thing will happen - black screen, with the rest of the stuff whirring, and if I try to shake it around nothing happens, I have to power off.

3. Occasionally when I power on, I just get the disk noise, a white screen, but no question mark, no apple logo, nothing.

4. Another time I tried to boot up with an external hard drive fitted and just got the chime over and over and over. Removing the drive solved this.

5. One time the screen just greyed out and a message in about 5 languages came up saying I needed to shut down my computer NOW.

This is not a hard drive fault - I saw that apple had extended the warranty on the hard drive because the question mark usually pops up for half a second at startup, and the nice guys at regent st London just whacked a new hard drive in, with a clean install of leopard (which was cool, I was previously on tiger at the time). I read something about resetting the PRAM (which I haven't tried), but I'm guessing it's the logicboard/motherboard. If this is the case, any idea how much it costs to fix? (My macbook is out of warranty). Figures in GBP appreciated but I can convert USD estimates. Is there any way I can try to blag a free motherboard and install from apple by exaggerating some symptoms?