Any ISP chip can be programmed direct via serial port ?
kind of yes.
Some Atmel chips claimed to be ISP are, but not from the serial port without major effort. Cygnal chips are JTAG and need a special cable. Other chips may have restrictions too as far as "programmed direct via serial port"

If, however you use the P89C51Rx2 or P89C66x from Philips or the AT89C51Rx2 from Atmel you can 'program direct via serial port'. Other such chips exist, those mentioned are the ones I can wouch for.

Have a good look at the P89C668, it has 64k Flash and the most RAM(8k) inside of any of the above chips.

Erik