I'm guessing the ATmega8 on the Olimex is running at 8 MHz (either with an external clock or the internal one). You can tell Arduino the clock speed of your board in the preferences file (Arduino - Home): search for "cpu". This will fix your sketches (e.g. the problem you mentioned of Serial.begin(2400) resulting in 1200 baud communication).
You could burn the Arduino bootloader to the chip to upload new sketches while it's on the board. Don't use the Arduino "burn bootloader" command if your board is running on an internal clock; it will set the fuses to expect an external oscillator and the chip won't work without one. If you're using the internal clock, you'll need to modify the fuses in your Arduino preferences file before burning the bootloader.
The bootloader was compiled with the assumption that your board is running at 16 MHz. If you have a board that's actually running at 8 MHz, you'll need to either recompile the bootloader (with the correct cpu speed defined), or change your serial.download_rate in the preferences file from 19200 to 9600.