I owned one of the Ati 9x series ..It has three ports ,One is DVi(white) ,One is VGA(blue) and the last one is S-Video out (black-circle)....
There is no HDMI port ..
You can use any monitor which has VGA or DVI port ...
Most monitors have options have multiple input options like HDMI/VGA/DVI ports ...
The S-Video out is for TV and is kinda pointless IMO ...
You will need a DVI cable for new monitor (if you prefer),or you can use the older VGA ,(No difference really)..
Just check your Catalyst drivers if your monitor resolution/frequency is supported !