Gameboy Development Forum

This site has only been up for a year now or so, and it's pretty new compared to other sites.
I would advice you to switch to assembly language, especially if you only have 32 kB at your disposal. But GBDK is using a pretty standard C compiler, so you could use pointers for it.
For example:

UBYTE *baseaddr = 0;
baseaddr[0x555] = 0xAA;

UBYTE defines the data type you probably want to use, an usgined byte. baseaddr is a pointer defined to point to address 0. baseaddr[0x555] uses baseaddr as if it was an array (Oh, the wonders of C) and writes to the position you want to write to.
The more traditional schoolbook example of writing to a given address is:

*(UBYTE*)(0x555) = 0xAA

But I personally like the former, and probably lesser known syntax better. (Mainly because you define the type in only one place)

In asm it's quite more easy. (You can also combine C and asm in GBDK and call asm functions from C and vice versa)

_write_somewhere::
  ld    a,#0xAA    ; Load the accumulator with 0xAA
  ld    (0x555),a  ; Write the value to the address in qeustion
  ret              ; Return

Of course things get a bit more complicated if you want to pass an argument to the function, but not much.

And lastly, there's already a project called GB-PIC that does this already that may be interesting to you.

And feel free to drop by #gbdev on IRC. (EFNet)

Use Google.
Google has this neat calculator feature, so if you enter 1/(150 ns) you'll get 6.6... megahertz, which is enough for Game Boy, and GBC in normal speed, but not GBC in double speed mode. (The normal speed of either hardware is 4 MHz, while the double speed is 8 MHz)