I bought a 98 xj8 and was wondering about programing an aftermarket keyless entry and another main key (black one). does anyone have any info on such?

 

What's wrong with the car's own keyless entry, buying a fob on ebay and programming it to the car is a painless cheap way, installing something to defeat the cars own security module isn't as easy.
A blank key can be bought and copied, the dealer then programs it to the car, you may as well buy 2 as programming is the same price. They can look up the key code also. There's cloning as well but I don't know about that.

 

Triple C (Jaguar Replacement Car Keys JAGUAR TRANSPONDER TIBBE KEY 98-99) can get you the key you need and program it.
Since you have a 1998, the dealer (for US models at least, not sure about others) does not need to do it, any place that can program a T5 transponder can program/clone a new key to your existing key.